"Small Vessel and Trike; Big Ruses"
Considering that the exact tricycle model that Abaddon, while bound to his vessel, rides in the Undervale hotel looks and works like a Big Wheel Tricycle, which is usually fitted for 3-7 years old children, in this AU, Abaddon's vessel is a 9 year old human boy whole is short for his age, being the height of a 6 year old despite being 6 years old, which is why Abaddon, bound to the vessel's body, is able to fit into that tricycle and use it just fine despite the vessel being 9 years old.
And to avoid the irritating and unnecessary scenerio of constantly correcting the public AKA humans who don't know that Asher, the human persona that Abaddon, in this AU, had adopted after Nathan legally adopt him as his adoptive son "Asher," is 9 years old and short of his age, since the vessel is 9 years old and short for his age, Nathan had the adoption system, and by extension, the government and public, believe that Asher was 6 years old at the time his adoption application was sent.
After Nathan died and Katherine, Ben and Esther moved into the Undervale Hotel bwcause Katherine inherited the hotel, and Abaddon's custody after Nathan's death, Abaddon, Katherine, Ben, Esther, and the other hotel ghosts, whenever they have the public know they exist, need to keep up the ruse that Asher is 7 years old and later 8 years old ("Asher" turned 7 years old, following his legal birthday, before Nathan's died, and turned 8 years old, still following his legal birthday, in the middle of Season 1, which was quite the adventure since the public had unintentionally pressured Katherine to give Asher a grand birthday since it was his first birthday after Nathan's death.
Since, in this AU, Nathan had legally adopted Abaddon as "Asher Freeling" before his death, after Nathan's death, Katherine, Ben, and Esther had to deal with living with Abaddon, who is legally part of their family through Nathan's legal adoption of him as "Asher," with Abaddon being Katherine's legal son and Ben's and Esther's legal brother, who is legally 3-4 years younger than Esther, who had lived as the youngest legal child of Katherine up until Nathan died and Abaddon's custody fell onto Katherine's shoulders (since Nathan, with Katherine's exasperated but accepting knowledge, made Katherine Asher's godmother, who is legally adopt him if he is no longer able to legally or physically be Abaddon's parent, like because he died, which ended up happening, or something else).
In this AU, Katherine is actually homeschooling Abaddon, to keep his demonic chaos at bay from the public and to keep up his "Asher" persona, much to her exasperated but accepting displeasure.
Nathan, Katherine, Ben, Abaddon, and the other hotel ghosts, if they have the public know they exist, carry out and keep up the ruse that Abaddon's name AKA "Abaddon" is just Asher's nickname, to match with his "impish" personality, so they could called him by his "demon self" name without raising suspicion, so the public, including the adoption system and the government, believe that "Abaddon" is just Asher's cheeky nickname.
What do you guys think of this "admittedly rather detailed" AU?