Jonas is a student at the Blackthorne Institute for Boys. He participates in the trip to the Gallagher Academy, along with Grant and Zach. The three of them are in the same grade level as Cammie and her friends.
Jonas is described as being skinny, wearing glasses, and having thick black hair. He also sports a buzz cut. Not many girls at the Gallagher Academy seem to be attracted to him, as he isn't the most attractive boy from Blackthorne, but Liz seems to have taken an interest in him.
Jonas is very smart and was said to be a finalist for Fieldstein Honor in his freshman year. Liz seems to have a crush on him because they share the same interests, both of them are smart, and when they dance, they never fail to step on each others' toes.
Jonas didn't have an official girlfriend in any of the books, but from the wink he gave Liz when they last saw each other in UWS, we can assume that he hasn't forgotten her and that he probably likes her as more than a friend. Jonas and Liz got to know each other better in CMH, when Liz was assigned to be his guide around the Gallagher Academy during the exchange.
Jonas is on the research and development track of study, like Liz, and isn't interested in fieldwork. If things turn out well, he will most likely become a successful technician of some sort for a spy agency.