He was still a child when Frisk fell into the dungeon, they met Frisk a few days after his fall. Before he became a royal guard, he wore a white jacket that was black on the inside and had fur on the hood. When Frisk fell, he was wearing a dark gray jacket with white fur, and a matching hat with a pompom. He really loves chocolate and tacos. X!Gaster, when he grew up, tried to teach him, Undyne and Papyrus how to fight in order to protect Frisk. At first, he and Papyrus did not want to fight, but to play with Frisk, but when people caused a lot of damage to Undyne, he and his brother began to train to protect Frisk. When it turned out that Frisk was not growing up (due to OVERWRITE), he and his brother continued to be friends for Frisk and somehow have fun. At some point, Chara and Frisk come up with a plan in which they use Cross to overthrow X! Gaster. Cross was overwritten several times by Frisk and Chara, but X! Gaster used RESET, rolling back the events to the last OVERWRITE. Frisk decides to show Cross what their life could be like without X!Gaster, but Cross refuses to change anything, which is why Frisk attacks Cross, gives him a scar on his face and gives him a nickname, after which a RESET occurs, and Cross is done forgets+Later, X!Frisk returns everyone's memories of the previous timelines to kill X!Gaster. Cross once again refuses to kill X! Gaster, for which his friends decide to beat him up as a traitor. Papyrus does not finish him off, which is why, a moment before X! Gaster’s death, Cross intervenes, kills all his friends except Papyrus, throws X! Gaster away and is going to take Frisk’s soul with the OVERWRITE ability in order to fix everything on his own. At a certain point, Papyrus stops Cross, for which Cross kills Papyrus and takes Frisk's soul, but realizes that the monster's soul is incompatible with the ability to REWRITE, which is why he is doomed to live forever in an empty world with the knowledge that he killed all his friends+After some time, Ink meets him. Cross befriends Ink, but when he learns that Ink cannot raise the dead, Cross quarrels with him. After some time, Cross is found by Error!Sans and steals his soul. Cross then takes half of Sans' soul to take control of the body. After some time, due to negative feelings, Nightmare finds him and makes a deal in exchange for the opportunity to travel around the AU. With the help of X!Chara, Cross takes the UNDERFELL!Snowdin code to rebuild his universe piece by piece. After this, he moves to UNDERSWAP, where he creates complete chaos on behalf of Nightmare. He is overpowered, classic Sans takes half of his soul back, but X!Chara takes SWAP!Chara's soul and gains control of Cross's body. At the end of the events of 0.4, X!Gaster brings Cross back, giving him the choice to join or go against X!Gaster. During the events of 0.6, Cross finds himself in the Omega Timeline, where he has to overcome his inner fears and hatred of everything, which leads to a battle with Killer and Nightmare along with Dream. After all these events, Cross chooses Dream's side, joins him on a journey through the worlds and helps him in protecting the positive from Nightmare.