What do you think is a toxic trait in a relationship?

Recently my cousin is having a real hard time with his girlfriend. They have been dating for 6 years she already met our family, and when we met her she was kind of shy but even after the years passes she didn't change with us well she didn't even try because she just didn't come anymore. They are still together and it wouldn't be a problem that she does come if they were just girlfriend and boyfriend but now they are fiancee because they were pushed to it by my uncle ( because for him my cousin is already too old) . But she still doesn't come to our family event even if my cousin come to all of hers she complains about having something to do every time once we even tried to changed the date for her but she still didn't come. But this is just the family part, with my cousin, when they figth about something she never apologizes despite her fault she always accuses him and when he tries to put some distance she calls him and say things like "I fell in the bathroom" or "I hurt my knee when falling from the bus" so my cousin always come back and come back.
Personnaly I think a relationship is a give and take interaction it doesn't work if it's only in one way so for me this is an exemple of a toxic relationship
What's your opinion?
What do you think is a toxic trait in a relationship?
Post Opinion