Uriel Ventris is Captain of the fourth company of the Ultramarines.
Official Canon Edit
Ventris is a very practical officer who, unlike his fellow Ultramarines, is doctrinely flexible enough to know when the Codex Astartes' recommended tactics shouldn't be followed to the letter. He has served in the Deathwatch, fought against the Tyranid invasion of Ultramar, as well as another invasion by the forces of Chaos. He also accidentally unleashed a shard of the Nightbringer during a mission to cleanse a Necron tomb world.
Cato Sicarius accused him of Heresy for breaking the Codex Astartes when he lead a leaderless Deathwatch Kill Team, and was forced to go to the Eye of Terror to destroy the Daemonculaba as penance.
TTS Canon Edit
Ventris confronts Marneus Calgar with his concerns about the Ultramarines' history somehow changing, wiping away previous transgressions and always putting them out on top. When Calgar started acting suspicious, Ventris tried to pry for more information, but was rebuffed.