He is one of the three "assets" sent by Alexander Conklin to kill Jason Bourne.
In Bourne's old apartment, Bourne senses someone coming towards him and Marie Kreutz. Marie asks Bourne if there's something wrong. Suddenly, Castel crashes through the window, shooting his sub-machine gun. He then take out his knife and engages in a one-on-one fight with Bourne, who counter with his ball-point pen. Castel was stabbed in the hand with the pen. Eventually, Castel was worn out in the fight, after Bourne had the advantage.
Bourne interrogate Castel and attempt to ask him who he was. As Marie searches Castel's bag, she was shocked to see wanted posters of Bourne and her. As Bourne goes to Marie to calm her down, Castel jumps out of the window to commit suicide before Bourne could ask him again.
As it turns out before the fight, the concierge who was talking to Bourne earlier was found dead downstairs.