Dieter is a young boy when found by Tenma to be under the care of a man named Hartmann. Upon meeting Dieter, Tenma notices that the young boy is covered in bruises. It turns out that Hartmann was trying to make Dieter into another Johan, a kind of leader that had not been produced from 511 Kinderheim before. Dieter later becomes more optimistic after being saved by Doctor Tenma from Hartmann's physical and mental abuse. He follows Doctor Tenma in his search for Johan, partially to prevent Tenma from becoming a murderer, but also because he is fond of Tenma. He later teams up with Nina, to give her moral support whenever her traumatic memories resurface as he was also subjected to similar abuse.