Admin: Some headcanons!
Fenrir is the one who gave Hel most of her education on their kind, given that their mother was dead and their father didn't discuss Jotunheim with them very often, Fenrir took it upon himself to tech his little sister. However, given how young they both were when their mother passed, their education is rather... incomplete.
Hel had a rather creepy habit as a child. Even at a very young age, she would try to tear people apart with what she knew about them, picking at any and every insecurity until they snapped and she knew she had gotten into their head. Now, why would she do this? She was never taught healthy coping mechanisms and was dragged away from her home, thrown into a realm where everyone seems to hate your family, and saw her father get married to a woman who, in her eyes, was attempting to replace her mother. She's dealing with a lot.
In case it wasn't obvious, Hel had serious daddy issues. There's a lot of resentment for not attempting to save her and her brothers, not telling her anything about her mother and barely visiting her since her imprisonment.