Grace is a girl from the Overworld that falls into the Pastel-ian world.
Toriel is pretty much Graces adoptive mother in the Pastel-ian world. Grace believes that Toriel is a good mother but is too overprotective. Toriel prefers to be called mom or mother but Grace insist on calling her Mum.
Flowey dislikes Grace and tries to manipulate her into giving him her colors. Grace insisted that she has none and that she has never had any. Seeing that she has a pure, colorful soul unlike him agrees with her trying to get her to "join him" He is sometimes a jerk to her and sometimes he complements her, either way, he is trying to get her soul. Toriel eventually comes along and gets Grace away from Flowey.
Papyrus and Grace get along very well and he ends up becoming her first friend. He likes to cook for her because he thinks that she knows the most about fine dining other than Undyne.
Grace Loves Sans very much but She afraid that she will one day lose him, even though they almost never fight and they get along very well also that he tells her he won't leave, her because he feels the same way about her.