Hégésippe was an ancestor of Twinsen. He was the last to hold the key to the Heir's room, before he was killed by the pirate LeBorgne. This story is scribed on runes at a cave near Twinsen's House.
Twinsen finds the lost key in LeBorgne's treasure in the Maritime Museum on Proxima Island, and uses it to enter the locked room in the cave.