Henrik van Boven is Phillip's BFF on the island of Curaçao. He's a Dutch kid who doesn't mind hanging with the West Indian people on the island. His country, Phillip tells us, was demolished by the Nazis (1.23).
Henrik's main purpose in the novel is to serve as a marker of how mature Phillip has become by the end of the novel. When Phillip returns to Curaçao, Henrik seems much younger to him:
I saw Henrik van Boven occasionally, but it wasn't the same as when we'd played the Dutch or the British. He seemed very young. So I spent a lot of time along St. Anna Bay, and at the Ruyterkade market talking to the black people. I liked the sound of their voices. Some of them had known old Timothy from Charlotte Amalie. I felt close to them. (19.40)
We don't get much more information about Henrik other than that, as Phillip finds more in common with the West Indian men and women once he returns to Curaçao.