Illiyan Nastase is an Astropath working for the Ultramarines.
Official Canon Edit
Illiyan Nastase was a character introduced in First Edition Warhammer 40,000 as the Chief Librarian of the Ultramarines. He was a half-eldar who grew up during the Badab War and was recruited as an astropath. Due to the reworked nature of the setting, both the idea of Eldar and Humans being genetically compatible, and the Imperium allowing Xenos in, were made highly infeasible, making Illiyan more or less retconned out.
TTS Canon Edit
While traveling in the Webway, the Ultramarines brought Illiyan with them to help them locate the Black Library. Marneus Calgar is very insistent to not allow him any screentime, possibly to hide the fact that they have a half-eldar in their employ.