Tea’s character made a lot more sense to me when I realized that until she befriended Joey, Tristan, and Ryou, Yugi was her only friend since childhood. It took a lot of work for her to like Joey and Tristan at first in the manga, and that’s partly because they started as bullies to Yugi–they didn’t really click until they saved her from Shadi’s test, risking their own lives.
I thought about her interactions with Mai and Kaiba some more and tried to imagine if it were me, and realized very quickly that I’d be very angry if someone I didn’t know insulted my friends, or someone I barely knew did the kinds of thing Kaiba did, especially in the manga where he tried to murder them. Tea’s big on first impressions and a very emotionally-driven person, and says what’s on her mind a lot. (Only time she doesn’t is when romance is involved XD)
We don’t see any of her home life in the series, and she seems to be an only child, so she hasn’t experienced the kinds of things Kaiba and Joey are going through in terms of sibling support, but she knows that Yugi is her friend and needs support–I think the more the series went on the more she realized she didn’t need to stand up for him anymore, since he’d grown enough to do that himself, but she could still cheer him on.
I kinda wonder if her parents were for or against her wish to become a dancer. I always assumed they were for it, but we’re not told either way.