@help-im-a-gay-fish She's done! Here's Celina! Her information and story are right below.
Name: Celina
Gender: Female
Age: 25
Appearance: She has patches of black on her bones(because of something that happened to her, I will elaborate in her background). She doesn't mind them and they don't hurt her anymore, though when she got them, it did hurt. Her eyelights had also been affected by the same thing that gave her the black markings. They were just a normal white color before, but now they're nearly black. But her eyesockets turned white, so they can be seen. Unfortunately, this made everyone uncomfortable with looking at her eyes, as it unsettled them. As for her clothing, she tends to wear t-shirts and sweatpants, and slip-on shoes. She also has a tail.
Personality: She is mostly calm, but has a fear of being forgotten.(This comes from the incident that gave her the changes in her appearance, she didn't have this fear prior to it) She is also kind of shy, but will open up once she gets to know someone. She also likes making friends, but has very few at the moment, those being Sans and Papyrus from an Underswap, and the Alphys from the Undertale universe she came from.
Backstory: She was a kind of assistant in the CORE, mostly running errands like making coffee or other drinks for the scientists, or delivering papers from one to another. She was friends with Alphys and always hung out with her on breaks, chatting and listening to her ramble about her projects. She also met Gaster and Sans, but didn't know them that well. That didn't stop her from being friendly with them, even exchanged puns with Sans during a break. But then it happened, the accident. The CORE had a malfunction that would have caused massive damage to the Underground, but Gaster and his crew managed to get it under control and shut everything down. Unfortunately, the room they were in was very unstable after all that, and Celina happened to be in the wrong place at the wrong time. She was in there passing along research from another scientist when the CORE malfunctioned, and she was too terrified to leave and hid under a nearby desk. That wasn't enough to keep her safe, unfortunately. The room collapsed, the floor crumbling underneath their feet, making them all fall into the bottom of the CORE .Sans was the only one who wasn't in the room, and Alphys was back at the Lab.
Gaster and his followers/assistants were scattered through space and time, but Celina, depending on who you ask, had gotten lucky. She wasn't scattered throughout space and time, but had ended up back at her home somehow. She wasn't unscathed, however. She was in immense pain for a few hours. When she was able to move with little pain, she noticed that she now had black markings that she never had before. She soon remembered what exactly happened. She then went to figure out what had happened to Gaster and his assistants. She passed by a mirror and noticed her eyesockets were different, it was like the colors swapped with each other. When she got to the lab, she first tried telling others that something happened to the CORE. But nobody believed her, as the CORE was now fine, any damage done reverted back when Gaster and his assistants vanished. Like it never happened, same for Gaster and his assistants. They also didn't remember her, despite her time working there. They were about to escort her out when Alphys came out, who seemed to be the only one who remembered her. By that time, Celina was having a mental breakdown, not believing that no one remembered her, and that there seemed to be no trace of Gaster, his assistants, or evidence that she worked there, or existed for that matter. Alphys helped calm her down and brought her outside, then asked why she was so distraught. Celina told her everything that happened, and what she remembered, asking if she knew what happened. Alphys, unfortunately, didn't remember Gaster, but definitely remembered Celina. They were such good friends, after all, and it seemed that what happened to erase the existence of Gaster didn't fully erase everyone's memories of her.
She also saw Sans again, and Sans seemed to have a vague feeling of knowing that he saw her at the lab, but they didn't interact as much after that. They occasionally run into each other and share puns with each other, she even got to meet Papyrus a few times. A few days later, she accidentally stumbled on a new power she had apparently gained from the accident: the ability to travel to other universes. Which is how she ended up in Underswap. The Papyrus there was the one who found her first, and she explained that she wasn't sure how she got there. He decided to help her after he saw how genuinely confused and lost she was. He told her about his theory of multiple universes, and that he thinks she came from one, as he'd never seen her around before. And there aren't many skeletons around. Some time passes, she makes friends with Papyrus and Sans, and she practices her power to see if she can try and control where she can go. She eventually found a way to get back to her universe, and said goodbye to the Swap brothers, but not without promising to visit them again soon.
When she got back, Alphys was the Royal Scientist and had already done the determination experiments. When Celina visited, Alphys had hugged her, crying. Celina asked what was wrong, and Alphys told her everything. While a bit horrified by what happened, she understood that Alphys was just trying to do her best, and wasn't entirely her fault. How was she supposed to know what would've happened? She comforted Alphys, and said she forgives her and that she wishes she was there for her. Maybe nothing would change, but she would have been there to help support her.
Current Day: Celina now travels the multiverse, but takes the time to go back to her home universe, and visit the Underswap she first visited. The human, she learns that their name is Frisk, didn't reset a lot, and has stuck with a True Pacifist route. Celina is aware of the resets, due to the accident(which made her aware of them), but doesn't remember what happens in them.
Other facts-
Magic: It was originally white, like her eyelights, but is now a black color. Has black bone attacks, she can also use black flames. but it surprisingly heals instead of harms.
Soul: It looks like a regular monster SOUL, but now has a black SAVE Star in the center, the outline shines a pleasant silvery blue. Her trait is Patience, with an underlying Kindness trait.
What the CORE accident did: changed her magic, changed her eyesocket and eyelight color, gave her black markings, gave her the ability to travel the multiverse, had the effect of her never existing when it came to others' memories(except Alphys), *made her aging stop entirely(she is now stuck at her current age, 25) though she isn't aware of this yet, gave her the fear of being forgotten(the mental breakdown she had, though it might be classified as a panic attack? Or something else).
How the CORE accident affected others: Sans does remember Gaster, but doesn't remember Celina as well as he does Gaster. He does have a vague memory of her being in the room that collapsed.(The reason he wasn't in the room but remembers her being there, is because Gaster used blue magic to move him out of the room, knowing he had low stats. He also locked the room. He didn't know she was still in the room when it collapsed, though. Neither did Gaster.)
Alphys only forgot who Gaster and his assistants were, as Celina was a very big part of her life at the time, and supported her a lot.
Everyone else(in her universe) don't know who she is anymore, and if they look at her for too long, they start getting a headache. They also find her eyesockets and eyelights unsettling, as they look somewhat unnatural.
She wears sunglasses for the sake of not making anyone uncomfortable. Though if someone wants to see them, she'll show them. The black markings don't hurt her anymore, and she doesn't mind letting them be shown. But she did have some moments of wanting to hide them in the beginning. She later realized, thanks to Swap Sans(who she calls Berry), who said that they look pretty cool, that she's much happier that she survived.