Life as Yara
Yara is the name Tristan takes when he deliberately transforms himself into a girl to join the School for Girls. As Yara, he squawked, twirled, and refused to talk (although she was overheard speaking to Dean Evelyn Sader by Agatha.) In exchange for being Yara, Tristan hid The Storian for Dean Sader in the spine of Sophie and Agatha's storybook.
Tristan is a prince with reddish-ginger hair and freckles. He is described as having a very similar appearance as Yara, with graceful limbs and longer hair. Also noted to still be very strong by Dot.
As An Everboy
In the School for Good and Evil, he never played a very big role, besides being one of the normal Everboys and Kiko's crush. He asked Kiko to the ball (and she accepted.) He doesn't play a very large role in the first book. At one point, Kiko goes to find him with hair dyed red, only to find that Tristan has dyed his hair blond to try and get Beatrix's attention. He copies Tedros and is told by Agatha to marry Tedros. Kiko says at the Circus of Talents that Tristan probably doesn't like Agatha because of this.
In the School, Tristan mysteriously disappeared from the School for Boys out of nowhere (see heading "Yara" above.) While he is gone, Tedros says he would be "the first one to hand the Storian over to a teacher," probably meaning that Tristan was more of a rule-abiding person when it came to authority and not much of a risk-taker. Yet, at the same time, he did take the chance of being Yara, due to how Aric bullied him for his rather feminine attributes, which was said to be part of the reason why he transitioned himself.
Yara is found to have been stabbed by Aric by Tedros and Filip, who is a magically transformed Sophie. As she dies, she turns back into Tristan, with her confession of joining the School for Girls being due to him finding it beautiful, along with his femininity. He then realizes Filip is really Sophie, confusing Tedros, and also confesses that Evelyn Sader knew who he really was, but allowed it. He dies soon after.