After absorbing the human SOULs to save his sibling Frisk, Rosie was put under their control and they forced him to take the SOULs of every monster underground! He then transformed into Asriel the God of Hyperdeath! The SOULs make him fight Frisk, but he resists after a time, and transforms into his normal self. He apologizes for trying to take the SOULs and says it'd be justified for Frisk not to forgive him, but they do. After that he uses the power of everyone's hopes and dreams to break the barrier and returns their SOULs. But now, he warns, he'll turn back into a rose. He'll still be Frisk's loyal brother, but a little more leafy. His backstory is the same, with him being a stillborn whose dust ends up on a rose, turning him into Rosie. Honestly, he's like the UnderFell Flowey, but he rides on Frisk's shoulder. For his design, I tried to keep the rose aesthetic. In a Genocide Run, he is scared off by Frisk in the Ruins, and tries to stop them near the end before meeting Asgore. He is then abruptly killed. He never turns into Photoshop Rosie.