Doc wiped his forehead with the back of a sleeve. It came away with Al-Thani's blood on it.
Al-Thani was an Orbital Drop Shock Trooper stationed on the UNSC Pillar of Autumn during the Battle of Reach and the Battle of Installation 04. He was a member of First Lieutenant Melissa McKay's unit. He was killed in action when the Covenant intercepted and boarded the ship. Judging by the way Valdez's face was covered in Al-Thani's blood, it was most likely he was critically wounded in battle, and the medic had attempted to save him but was unable to.
- Halo: The Flood (First appearance)