Life as Yara
Yara is the name Tristan takes when they deliberately transform themself into a girl to join the School for Girls. As Yara, she 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 an everboy with reddish-ginger hair and freckles. Tristan 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, they never played a very big role, besides being one of the normal Everboys and Kiko's crush. They asked Kiko to the ball (and she accepted.) Tristan doesn't play a very large role in the first book. At one point, Kiko goes to find them with hair dyed red, only to find that Tristan has dyed their hair blond to try and get Beatrix's attention. Tristan 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 they are gone, Tedros says Tristan 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, she did take the chance of being Yara, due to how Aric bullied them for their rather feminine attributes, which was said to be part of the reason why she transitioned herself.
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 her finding it beautiful, along with her femininity. She then realizes Filip is really Sophie, confusing Tedros, and also confesses that Evelyn Sader knew who she really was, but allowed it. She dies soon after and a large firework is set off