AbsenTale, a Gaster AU
This AU is still one of my favorites, and one I was very proud of even before revamping it. The basic idea is that instead of Gaster being erased, Sans and Papyrus did, and Gaster has to fill the roles of both brothers in the game to still hit all of the important plot points. It is so named because Sans and Pap are "absent" and must be replaced. This post is just what happens in the Underground, as I have some history for Gaster during the human/monster war, and it ends before Frisk falls, as I'll have a different post explaining how Gaster fills those roles, as well as some fun stuff that happens once they get to the surface.
But for now...
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THE STORY SO FAR
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As the defeated monsters fled Europe across the ocean, they landed in what would eventually be known as North America, with the humans hot on their heels. They didn’t have time to establish themselves before they were sealed under Mount Ebott.
Gaster was among the refugees, having just lost his whole world and in a dissociative state, until after the Barrier went up and he slowly clawed his way out of his depression by embracing his hatred of the humans that took everything from him. He joined the Royal Guard and did quite well, befriending another young recruit, Gerson the Tortoise, but realized that feeding his hatred was more self-destructive than anything else and quit, instead turning his attention to improving the lives of the surviving monsters and eventually attaining the title of Royal Scientist. As both skeletons and tortoises have longer lifespans than most monsters, as do Boss Monsters, Gaster and Gerson maintained their friendship over the decades/centuries (however long they were down there), and Gerson was more than happy to test all of the things Gaster invented.
At some point, Asgore approached Gaster about taking on the role of Judge. He accepted.
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NOTE: I'll go into more detail about the Judge later, but in short, a Judge is "set apart" to have certain special abilities to help, well, judge. The exact powers and dominions of a Judge are obscured as much as possible, since keeping all of that completely hidden would be a futile effort, but the actual identity of a Judge is a carefully guarded secret. No one is supposed to know who the Judge is, or how many Judges there are at a given time, so they can't be targeted for favors or retaliation among other things. A Judge is also supposed to accept the veil of anonymity to help put aside their own biases while performing their duties impartially, and separate themselves from whatever Judgement they are compelled to carry out.
Asgore and Toriel were filling the role themselves until everything settled in the Underground, but now they wanted to be able to focus on the future rather than looking back to punish past actions. They picked Gaster for several reasons, including but not limited to; he's smart enough to understand the nuances of Passing Judgement; he's strong enough mentally and physically to carry out Judgements; he's already fair and logical, feeding into the required impartiality well; he hadn't aged since they'd met him so they knew he'd be around for a long time.
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As Royal Scientist, Gaster was tasked with a great many responsibilities, such as making the Underground at least livable, if not comfortable, and finding ways to escape the Underground. After Asriel’s death, he was also asked to find a way to strengthen the monster race enough to survive the war Asgore had declared on humanity. After an epidemic of monsters falling down, Gaster decided they needed a way to rebolster the population, without forcing monsters into parenthood that didn't actually want it, and turned to cloning. For the initial experiment, Gaster used his own soul as the template and supplemented the lack of partner by using the human souls they’d gathered by then, but if it worked, the goal would be to essentially take "samples" of every living monster to create "genetically" diverse children in mechanical incubators that would likely be raised in the equivalent of an orphanage, group home, or permanent dormitory.
For now, though, Gaster was determined to see these dozen clones as real people and not just experiments, so he named each clone and considered them his own children, no matter how they came into the world. Unfortunately, he hadn’t considered the possibility that they wouldn't make it, of his inability to save them, and his psyche began to unravel as he mourned each death, which piled on top of the grief from before the barrier that he’d never fully addressed.
Sans and Papyrus were the only survivors.
For just over a decade, the brothers were the only thing that kept Gaster’s cracked psyche from fully shattering, hiring Alphys as an assistant to help pick up the slack while he raised them. Despite technically being the same age, Sans took on the role of ‘big brother’ for the slightly less mature and more naïve Papyrus, and Gaster allowed them to help him around the lab if they wanted to; it gave him an excuse to be around them more, to appease his own separation anxiety, and allowed them to feel helpful and accomplished. Alphys adored them both and became something of a big sister or "cool aunt" to them.
Things were looking up, until tragedy struck as an experiment failed catastrophically, wiping not only Sans and Papyrus from existence, but any record of the other failed clones as well; it was as if he had simply decided to not go forward with the clone experiment all those years ago. Not even Alphys remembered them. With nothing to ground him anymore, Gaster was driven past his breaking point and resigned as Royal Scientist, dumping the title and responsibilities on Alphys’ young, ill-prepared shoulders, and spiraling into a deep depression.
As one of his closest friends, Gerson was very concerned by this sudden change in Gaster. Gerson didn’t remember Sans and Papyrus either, but he believed Gaster wouldn’t lie about something like this, and there was no way Gaster could fake this kind of reaction to losing them; he generally hated drama and definitely wasn't a good enough actor to pull it off. While Gerson knew Gaster could survive on his stockpiled earnings, he also knew Gaster would waste away with nothing to do, no goal to achieve; he had always been a bit of a busy-body and was now in very real danger of falling down himself. As Captain of the Royal Guard, Gerson pulled some strings and managed to secure a sentry post for Gaster in Snowdin Forest, the farthest he could physically get from both the Hotland Labs and his former residence in New Home, as both locations were now very triggering. As they had continued to spar fairly regularly over the years, Gerson knew that Gaster was physically capable of the requirements of being a Guard and that sentry positions weren’t very taxing anyways, especially so far into the forest. Gaster could continue to do something, or at least feel like he was doing something, with minimal responsibility or repercussions for failure, and still allowing himself the time and space to grieve.
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Time inevitably marched onward and many things happened over the next few years:
Alphys conducted her DT experiments, creating both Flowey and the Amalgamates
Mettaton was given his original, calculator robot body
Gerson retired, appointing his protégé Undyne as Captain in his stead
Gaster discovered Toriel behind the Ruin’s door, though he didn’t know it was her at first
The residents of Snowdin became used to Gaster and his... oddities, especially since none of it was the ‘dangerous’ kind of crazy
Other things probably happened as well, and not necessarily in this order.
Expanding on Gaster’s oddities, he continued to suffer from depression and bouts of "insanity" where he would see "ghosts" of Papyrus and Sans. Scientific evidence of their existence was "inconclusive" at best, and no one else could see or interact with them, but Gaster couldn't let go of the feeling that they were real. He did consider the possibility that he was just crazy, that he really hadn’t gone forward with the clone experiment all those years ago, that he'd been living in a delusion of raising kids that never existed, and the "ghosts" were just his mind clinging to a fantasy as he "transitioned back" to reality. However, after many tests on his own mind/psyche, Gaster firmly believes that he is perfectly sane and that Sans and Papyrus do indeed exist. Somehow.
He couldn't bring himself to dive into the science of the failed experiment yet; his hand shook, his chest tightened, he couldn't breathe, he couldn't think...
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As the new Captain, Undyne wanted to run the entire Guard through a boot-camp of sorts, in groups based on current assignments, for about a week each. There were several reasons for this decision, including, but not limited to:
Reasserting Dominance - She wanted to prove to each and every member of the Guard that she deserved this position in order to avoid insubordination and head off any attempts at mutiny before they started. Gerson had been Captain for a long time and change was always difficult.
Reintroducing Herself - She did know most of the Guards already, but that had been as their equal, not their superior. She also wanted to become more familiar with the team dynamics of the Guard Units she'd had less contact with and identify their strengths and weaknesses.
Reassessment of Positions - This would be a trial period for any new placements, if any guards wanted or needed to be reassigned to different units or change duties (like RG 01 and RG 02 wanting to be assigned as partners). She would also be making sure that every member of the Guard was up to snuff and met her standards.
Undyne would never admit it out loud, but the ‘reassessment’ was really just an excuse to try to get rid of Gaster. She hated the guy. She cited that he was a ‘drain on resources’ since she knew he got a larger paycheck than anyone else, even her (because he's the Judge, which she's not supposed to know), and she thought his depression and on-and-off alcoholism (or magic equivalent) portrayed an unprofessional image for the Royal Guard in general. More than anything, though, she blamed him for Alphys’ current mental state, and she’s not exactly wrong. Alphys and Undyne had been friends for a while before ‘The Incident’ and Alphys hadn’t responded well to the pressure of being the Royal Scientist.
During his week of training with the K-9 Unit of Snowdin, Undyne was far harder on Gaster then she had been on anyone else, but he continued to perform well enough to pass inspection. He was often sarcastic but still compliant, and even humble, never showing off but only ever doing the absolute minimum necessary to complete his given task. While grudgingly impressed that he could calculate exactly how little he could get away with, it only proved to frustrate Undyne further, as she couldn’t fire him due to underperformance; Gaster simply found this mildly amusing.
At one point, she tried to boot him for "ruining team dynamics" since he wasn’t a dog like the rest of the Snowdin K-9 Unit, but he smugly proved her wrong by displaying a modification to his natural Purple Trap Magic. It usually manifested as hand-shaped magic constructs that would grab and hold a target, limiting their movements, but they could also be applied to his own body like armor, which he then manipulated to make him look superficially like a werewolf, complete with digitigrade legs, giant clawed paws extending from his arms, and even a prehensile tail for the fun of it. The look was completed with a helmet styled after his signature Gaster Blasters. While Gaster had kind of made it up on the spot to annoy Undyne, the K-9 Unit were very impressed and suddenly much more interested in getting to know him. Gaster hesitated letting them in, as he'd been emotionally distant from almost everyone for decades, but the ghosts of Sans and Papyrus convinced him to give the dogs a chance. To his surprise, Gaster found that he didn't regret it.
Now desperate for any reason to have him canned, Undyne decided to give Gaster a private training session and said some very hurtful things in an attempt to get him to stop holding back, to see what he was really capable of when he actually applied himself. To see if he really deserved the praise Gerson had always given him. She eventually struck a nerve by bringing up the erased skele-kids, which she’d heard a bit about from Alphys, and finally got the reaction she’d been looking for; Gaster snapped and threw every ounce of magic at his disposal at her, eventually falling into something like a berserker rage in his new werewolf form. Undyne slowly began to regret her decision.
Luckily, Gerson had been on his way to see how training was going and the sounds of conflict spurred him to move faster, lobbing his hammer at Gaster’s head as he loomed over Undyne; while not quite knocked unconscious, he was stunned out of berserk mode and the constructs dissipated as he collapsed. Gerson helped both injured combatants inside Undyne’s house, tending to their wounds while sharply berating the pair. Gaster, however, barely seemed to acknowledge his existence, let alone hear his reprimand, fully dissociated now that the rage had died down. Undyne was told the full story of the skele-kids and finally started to understand the situation from Gaster’s perspective, not just Alphys’, and just how broken Gaster really was. With a newfound fire, Undyne asserted that Gaster would be allowed to retain his position as Snowdin Forest Sentry on the condition that he continue training with her personally once a week; partially to help "rehabilitate" Gaster, but also to hone her own skills against a powerful opponent now that she knew what to expect.
So he’s more useful to the Guard, of course! Totally not because she actually cared about his well-being or anything. She still blamed him for Alphys’s current mental state, after all, even if he did show remorse for leaving her to be crushed by the responsibilities.
Gaster desperately needed structure in his life, which started with Undyne’s weekly training, then extended to a "desensitization program" to help control the panic attacks he tended to get the closer he got to the Hotland Labs. He would travel through Hotland as quickly as he could in his werewolf form, channeling his ‘nervous energy’ into something productive, like running as fast as possible. He would then have a calming cup of tea and talk to Asgore about his troubles before running through Hotland again. Once back in Snowdin, he would take a walk through the forest to calm down, often talking to the "nice lady behind the door" for a while. At first, he could barely handle doing this once a month, with the goal of making them weekly as well.
It took a while to put two and two together, but Gaster did eventually figure out that Toriel was behind the door; she never mentioned it so neither did he. As he would go back and forth between the two, though, Gaster found something he could throw himself into; repairing their relationship. It had been a while since he had felt a desire to do anything, let alone something productive, and he relished the feeling. It still waxed and waned with his depression, but he considered it progress.
After wallowing in his grief for so long, Gaster was finally starting to see a light at the end of the tunnel. He had fully accepted that Sans and Papyrus were real, that his memories were real, and decided to start looking into the failed experiment, despite how triggering it still was. He had expanded his support system from pretty much just Gerson, and maybe Asgore (and Toriel before she left), to Undyne and the K-9 Unit, hesitantly reaching out to other Snowdin residents, and even reconnecting with Alphys. He actually had goals, even if they were self-centered around his own mental health. With all of this, he managed to keep himself from falling back into alcoholism, and eventually made the classic promise to Toriel to protect the next human to exit the Ruins, both for Toriel’s sake as well as his own. Despite his still smoldering hatred of humans, he couldn’t stand the thought of being the cause of another child’s death, even a human one.
Gaster is aware of the existence of resets but doesn't know they are connected to Flowey (or eventually Frisk) and doesn’t remember them any more than anyone else. While most people chalk up to déjà vu, Gaster knows better, though he can’t really do much about it.