Karsten is described as "disheveled-looking." He is tall and gangly, with both his eyes and curly hair a dark, coffee colored brown. He has a pronounced nose and tanned skin. He often wears casual clothes, like mostly hoodies and sneakers.
Karsten is an introverted, self-conscious indiviual. Karsten claims that he was never bullied in school, just ignored. He usually dislikes speaking to other people, seeing as how he has a tendency to lose his cool when someone does something unusual or that he doesn't agree with. He is shown to be extremely violent whenever he is under stress, under pressure, or even when frustrated. Karsten is a fairly perceptive friend, but doesn't seem to know his limits or boundaries in conversations. He can be highly confrontational toward others, even getting physical at his breaking point. He's also very blunt toward people like Dilshad and Milly about their issues, and tends to snap at people when they patronize him.
Despite his hot-headedness, Karsten is a clever, imaginative and insightful individual. His quick thinking and flexibility aid him tremendously in adapting to the numerous and uncanny challenges presented by the Nightmare Game and his imagination and incentives awards him a frightening aptitude for climbing walls. Additionally, his insights allow him to encourage other dreamers to be at their best and to not give up.
Vincent was born in Azusa, California. A few references are made of Vincent's mother; she left a man she was dating for someone else, who is Vincent's biological father. Karsten's father left shortly after his mother got pregnant with him, and disappeared from Karsten's life forever, before he was born. Karsten lived in Azusa with his mother until he was two and a half years old. She moved them to Albany, New York, to live with his mother's old highschool friend, Vincent. His mother mysteriously disappeared a little over a year later, leaving Vincent to adopt Karsten.
Karsten doesn't remember much about his mother, and therefore doesn't really care to go out and find out what happened to her. When he was six years old, Vincent made him sign up for an after-school club in hopes of him making friends. There he met Milly, a mischievous girl his age who found pleasure in tormenting the other children and the instructors. He tries to avoid her, until he's put in a three-person group with her and a heavy-set boy called Dilshad. Milly began to treat Karsten differently from how she treated the other kids, pushing him out of his comfort zone and getting him to join competitions and making him feel good about himself. He grew to have a mutual friendship with Dilshad, based on their unpopularity and their fascination with Milly. Inevitably, he, Dilshad, and Milly become close friends, especially when Milly began attending the same elementary school as the boys.
In the fourth grade, a new girl by the name of Perdita moved to Albany, specifically to Karsten's neighbourhood, and joined their class. She introduced herself to Karsten, which ended in failure when he complained that she smelled bad. The two had a rocky start, especially when Milly would bully and make fun of her. When Milly moved away about a year or so later, however, Karsten and Dilshad wound up hanging out with Perdita outside of school. Karsten and Perdita grew especially close, hanging out after school and doing their homework. He eventually persuaded her to join the wall climbing club with him, though it didn't last more than a couple of months when the building and program were shut down. Perdita eventually stopped speaking to him during school, focusing more on clothes and makeup and becoming popular.
A couple of years later, Karsten ran into Milly at a bus stop. He convinced her to hang out with him, and they decided to break into the abandoned Eden Tree building and climb together. But when Milly's emotions caused her to lose focus and fall off the wall, breaking her arm, the two had a fight and she left him alone in the building.
A couple days later, while he, Dilshad, and Perdita were all having a sleep over at Karsten's house, they got a phone call that said Milly had "committed suicide."
A week later, after days and days of begging Vincent to let them go find Milly's house and see her family, still to no avail, Karsten ran away one afternoon and took a bus to where Milly lived in Newark, New Jersey. Unnkowingly to Karsten at the time, Raziel had followed him and allowed him to travel back in time to Milly's true time of death so that he could meet Milly's father.
The visit didn't last long, and soon Karsten was back at the bus stop where Raziel sent him back to his original timeline.