Undertale Yellow; Chartreuse is an Alternate Universe of Undertale Yellow (2023) which is a fan-game of Undertale (2015). The basic idea behind it is a typical role-swap Alternate Universe, except instead with the idea of “How could these pre-existing characters end up in different positions later in life compared to their original starting point?”.
I suppose the general inspiration for this comes from the idea of Underswap or Storyshift, Undertale Alternate Universes which switch different characters into different roles, combined with the sequel/continuation setting of Horrortale, an Undertale Alternate Universe set after an Undertale Neutral Ending. The name Undertale Yellow; Chartreuse, shortened to be UTY;C, was chosen to both follow the naming conventions of other Undertale Yellow Alternate Universes (being different shades of yellow) and because “chartreuse” is famously known for being a name which does not feel like it matches its colour, something fitting for a role-shift AU.
Kanako - Kanako had lived all her life in the Ruins with little contact with other Monsters outside Ceroba. She always had a curiosity about the larger underground, but was discouraged from it. She snooped around her mother’s belongings one day and found some old tapes (Chujin style). The tapes depicted a young Human being hidden in the Underground by some unfamiliar Monsters (Starlo, Martlet, and Dalv). Vaguely knowing about the need for Human souls to break the barrier, she set out on her own to find and capture this hiding Human to become a hero for Monsters. She’s assisted by her secret best friend, Flowey.
Clover - Clover fell into the underground much earlier than in canon, falling into the Dark Ruins without Toriel or Flowey knowing. Instead of letting Clover advance on their own, Dalv decided to go with Clover to Snowdin to protect them. Dalv and Clover met Martlet and Chujin (prior to the events depicted in the Zenith Martlet flashback). Chujin is apprehensive about Clover but Dalv and Clover befriend them both. Chujin led them to the Dunes. Clover and Starlo meet and immediately grow attached to each other, forming a strong father-child relationship. Eventually, due to everyone growing complacent on keeping Clover hidden, Clover and Chujin end up getting caught by a Royal Guardsmen in Waterfall, an area both Clover and Chujin are unfamiliar in. Chujin defended Clover in vain, they both died in the ensuing fight. Clover ends up haunting Kanako in a similar manner to Chara does to Frisk, helping her along her journey.
Ceroba - Ceroba is married to Chujin, but both elected to not have children until Monsterkind’s future was more secure and hopeful. She met Dalv and Clover through Chujin after they arrived at the Dunes. She became friends with Dalv and acted as an aunt to Clover. Loving to help take care of Clover, Ceroba and Chujin decided to have a baby of their own. However, distraught by Chujin’s and Clover’s simultaneous deaths and remembering Dalv telling her about his time in the Ruins, Ceroba fled to the Ruins with her baby, Kanako. She elected to raise Kanako away from the larger Underground, not wanting Kanako to be raised in a place which would condone the murder of a caring Monster and an innocent child. Ceroba is lonely and still deep within her grief, but focuses all her attention on raising Kanako and keeping Monsters out of the Dark Ruins.
Axis - Chujin, having fewer stressors and responsibilities since he was childless, had the time and energy to fine tune the invention of Axis. That led to fewer faults in his programming when presented to Asgore and the Axis Project being green-lit. Axis was constructed and placed in Snowdin to be trialled due to the lack of official Royal Guardsmen being in the Snowdin area. Chujin had control over Axis, but rarely used these controls due to an appreciation for Axis’s developed individuality. Martlet and Axis became close because of their similar relationship with Chujin. Initially he tried to capture Clover on sight, but Martlet convinced him to hold off. He became friendly with Clover as they visited Axis in Snowdin. Axis was deeply saddened by Chujin’s and Clover’s deaths, losing all of his closest relationships shortly after when Martlet moved to the Steamworks for work. He knows about Ceroba’s location in the Dark Ruins but purposefully misdirects people from her location to honour his creator. Axis runs autonomously in Snowdin as an asset of the Royal Guard, although none of them know how to operate him.
Dalv - Dalv was attacked by the Blue Soul Human when he was alone in the Snowdin forest, leading him to run off to the Dark Ruins in fear. He was given Coffee Beans by the Honeydew Resort residents when they heard about the attack, leading him to learn how to grow Coffee Beans. He eventually left the Dark Ruins with Clover and moved to the Dunes. He grew close to Starlo and his family as Clover did, bonding over an appreciation of farming. He was heartbroken by Chujin’s and Clover’s deaths as well as Ceroba’s and Starlo’s grief. Refusing to fall back onto his isolationist habits Clover helped him out of, Dalv took over Starlo’s role as leader of the Wild East to give him time to grieve. However the longer he takes on this role the worn down he becomes from his new found duties, but the Feisty Four help him out. Dalv does have a hunch on where Ceroba is, but he has no spare time or energy to pursue his suspicions. As a result of his leadership the Wild East has gained a Western Gothic aesthetic.
Martlet - Martlet never ended up joining the Royal Guard due to meeting Clover earlier on. She instead hopped between the Dunes and Snowdin while doing odd jobs for money in between, such as helping Axis with puzzle maintenance or Starlo in constructing his new house. As a result, she decided to become an independent contractor. After Chujin’s and Clover’s deaths she elects to remain in Snowdin to find company in Axis. She eventually gets hired by Asgore to rework the abandoned Steamworks into more living space for Monsters. Asgore wanting as few people to know about the old Steamworks as possible resulted in Martlet becoming exclusively responsible for the project. Eventually she had to move to live in the Steamworks just to make any significant progress on the project. The project was named the “Steamworks Reconstruction, Repurposing, and Reassimilation Initiative” or “S3RI”.
Starlo - Starlo was closer to Ceroba than he was to Chujin naturally, but ultimately was amicable with him and could get along with him reasonably well despite differing personalities. However, after meeting Clover through Chujin, the two became much closer. Starlo immediately became close to Clover when he met him and their friendship soon turned into a father-child relationship. He made a new house, with help from Martlet and Chujin, a little outside of Starlo’s Family's Farm and the Wild East so he and Clover could live together. He was greatly devastated after finding out Chujin’s and Clover’s deaths, the feeling being compounded by Ceroba’s disappearance shortly after. Now he only ever leaves the withering away house for the barest necessities. He has a hard time being around Western paraphernalia, the exception being Clover’s Hat and Gun which he keeps on him constantly. His relationship with the Feisty Four is estranged but they're still friendly with each other when they do see each other. Starlo is extremely lonely without Clover and Ceroba, by far the closest people in his life, and finds little direction in his life without them.
