Spoilers: I'm going to stick to Basics for a while, even though they contain spoilers...
AKA: Diet (pronounced "deet"). Dietrich is pronounced "DEE-trick".
Age: ~30 in Trolls (~23 in year 0)
Looks: 4'3" (~human 8-year-old), blond hair with light blue streaks, green eyes - looks like Aldrich.
Character: Genius. Too well-behaved, sweet, and high-strung. Clingy at times, but adorable.
Special skills: Fire healer. Scholar. Green thumb.
Family: Only child of Aldrich von Bielenfeld and Glynda von Wincott, born about the 70th year of their marriage. Glynda was a first cousin of Suzanna Julia von Wincott, whom Aldrich disliked.
After Aldrich and Glynda's marriage, concerns arose about Glynda's behavior, and Aldrich decided to wait before having children. These fears panned out - Glynda was mentally ill, and her condition progressed with age. This was attributed to aristocratic inbreeding, and the couple were 3rd cousins. But both really wanted children, and after 70 years of marriage, they had just the one - Dietrich.
Dietrich's father Aldrich and grandfather Friedrich were his reliable parents. When Glynda was "doing well", she lived with them at the Castle, but it was like walking on eggshells. More often than not, Aldrich kept her hospitalized. In year 4 (between stories), Aldrich finally decided to divorce. Glynda ended up committing suicide. So this attempt to make things better for Dietrich, made them worse instead.
The family hopes that Dietrich's nervousness, and anxiousness to please, are due to Glynda's influence, and not any inherited mental illness. He's an angelic boy - too good.
Dietrich was named heir to Bielenfeld when Aldrich became Lord (WTB). But under Bielenfeld law, that isn't binding. Dietrich is cross-fostered with Trenton von Gratz. The boys tend to vacation with Trenton's family, and do schooling with Dietrich's, though that may change as Dietrich's home life improves. Dietrich idolizes Manfred's son Efram, who treats him as a kid brother.