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.)
Yara is found with a knife wound in her throat by Tedros and Filip (a magically transformed Sophie). As she lays dying, she returns to her original form, that of Tristan. Tristan had used one of Merlin's spells to turn into a girl. Tristan states that he felt irresistibly drawn to the School for Girls because it was so beautiful (whereas the School for Boys is closer to its original design as the School for Evil). The dying student realizes Filip that is actually Sophie, which confuses Tedros at the time. Yara/Tristan confesses to Filip/Sophie that Evelyn Sader knew he was using a magical disguise, but allowed him to remain in exchange for hiding the Storian.
Tristan/ Yara dies as a girl because Aric believed he was killing a girl during the Trial near the end of the second book, A World Without Princes. His/her death convinces Tedros that, despite his previous threats to kill Sophie, he, in fact, doesn't wish to murder anyone.
Tristan is a prince with reddish-ginger hair and freckles, who loved to dance and was teased because he was typically more feminine than other boys. From his behavior , it can be implied that he was transgender.
As An Everboy Edit
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 and 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 into Yara using Merlin's spell.