For Unknown reasons in everyone's memories they are only known by the name Frisk but their true name was in fact Chara. They were the first human to fall down into the mountain. They were found by Asriel and taken in by Toriel and Asgore. Soon they fell ill and died. Asriel wanted to grant their last wish, he fused with their soul and took them to their old village to see the flowers.
They saw Toriel as their mother and were very happy to to apart of the family
They saw Asgore as their father and enjoyed hanging out with him in the throne room and playing. They would enjoy having tea with Asgore.
Asriel was like a brother to them and enjoyed his company they would draw together and explore the underground together
It is hinted at that Gaster knew Chara's true name and seems to be the only one that remembers the fact that their true name wasn't Frisk.
If a genocide run were to happen, Sean would meet the spirit of Chara just like in Undertale. Chara would wake up and praise your killing skills and want to erase the world. Refuse and the jump scare happens and crashes the game.