Tomilin grew up on farm on the bioship with his father and mother, living in a cabin near the edge of the ship's ecosystem. His parents were followers of the Word of Dorahl, but in his youth, Tomilin began to doubt the veracity of his parents' religion and quietly joined the Reformers, a group of philosophical dissidents.
After an away team from the USS Orville enters the ecosystem of the Dorahl Bioship in late October or early November 2419, Captain Ed Mercer stuns Tomilin's father with his PM-44 and the team ask his mother questions regarding their ship and their beliefs.
Tomilin is more direct and asks where they came from. Ed tries to explain that the bioship is about to collide with a star, but only confuses Tomilin. He suspects that the others are from "the Beyond" (the name for the world outside of the ship's ecosystem), and takes them to Kemka, leader of the Reformers.
Tomilin accompanies Kemka and the Orville's away team during the overthrow of the religious despot Hamelac. He is last seen entering the Bridge of the bioship, marveling at space and watching an ancient recording of Jahavus Dorahl.
Tomilin has a very relaxed and logical demeanor. Even after Captain Mercer had stunned his father, and after Isaac and Doctor Claire Finn explained the greater universe, Tomilin appeared to take everything in with only subtle amounts of confusion and shock. Though, he might've expected their claims to be true as he was a member of the Reformers and supported the Dorahlians exploring outside the Bioship ecosystem.