About the main character
It's a 20 years old female who's owned by W.D. Gaster. She has been with him since she was 10 and was pretty much raised with Sans and Papyrus.
She possesses a green soul, kindness, and the ability to use healing magic. Humans cannot use magic without a 'medium'. Medium may be an object or an act. Main character's medium is music. She either plays a flute or sings. Singing has a weaker effect than playing. Her magic does not work on herself.
Her magic abilities were the reason why Gaster chose her to guide. He wanted to study and provide proper guidance to her, as he felt that some monsters could use her ability as a means to gain fortune or popularity. (Which is not illegal if it doesn't cause any damage to the human.)
She can understand and write Wing Dings, but is not fluent to speak it. (To be fair, she ruins the accent, poor baby)
She takes care of a golden flower that she named 'Flowey'. But Flowey is not alive. At least, not yet.
She co-exists with Frisk, who's 10 years old in this Universe and is owned by Sans(more details about this on the Gaster family post) and they have a close sibling-like relationship.
Sans was her first (and only) kiss. After said kiss, he became cold and colder to her and nowadays, their relationship is of mere acquaintances. She never inquired why he changed his approach. Instead she accepted and noved on. Feelings lingered.