Kai was introduced in Ring of Judgment as an engineer's apprentice. He is doing a lot of overtime to make ensure he can take care of both himself and his sister, Tilia. He tries to save up money so they can have a doctor look at Tilia's far-sightedness.
Ring of Judgment
Accepting the request to solve the case, Toval Randonneur, Estelle and Joshua Bright infiltrate a nearby fortress where they find Kai under control of an artifact, the ring of judgment. Unaware of his actions, Kai controls an earth-based monster and attacks the bracers. Through a clever strategy Joshua manages to remove the ring from Kai's finger and he regains consciousness a couple of seconds later. Together with Tilia, he returns to the town while the bracers take care of the mission's aftermath.
Some time later, when Kye gets home from work, he finds Tilia waiting alongside Toval and Ein Selnate. Toval informs him that a client has requested that all funds go to the treatment of Tilia's eyes. Overwhelmed by the generosity, Kai tears up and Toval consoles him.
In Trails of Cold Steel II, Memoir: "The Remote Town of Alster", Kai makes another appearance as Class VII pays a visit to Alster. Upon delivering orbments to the orbal factory, they are greeted by Kai as the one responsible for the factory at that moment. Surprised by his young age, he reassures them it is nothing out of the ordinary for him.
The conversation is interrupted by the roar of a cryptid. Still feeling guilty over the ring of judgment incident, he swore to himself that he would do whatever he could to help if anything like that happened in Alster again, so now he considers it his duty to assist the class by guiding them into the mountains.
After the cryptid is defeated, they see Alster being attacked in the distance and rush back down again. During the battle in Alster, a Panzer Soldat dashes into a building Tilia was near. The falling debris is fortunately stopped by Altina Orion and Kai runs over to his sister. Together, they watch the battle that unfolds before their very eyes.
After the matter is resolved, Kai and Tilia see Class VII off. Knowing that Tilia has been working on a story and that she wants to be an author, he asks her what the papers under her arm are to trigger Toval's curiosity. She is surprised that he figured out her poorly kept secret and tells the two that it is a work in progress, but also admits that she wants Kai to read it most when it is done.