T. Ryan was a scientist on Mars. He was one of the few UAC personnel to survive the initial demon invasion, although he is somehow oblivious to it. Despite this, he correctly suspected that "all the weird things" were originally from the Delta Labs. Ryan and his partner were on the Monorail tram when the demon invasion occurred. His partner went to investigate but never returned, instead being transformed into a Commando by Betruger.
When the player activates the Monorail tram in Doom 3, T. Ryan suddenly opens the rear of the tram, revealing himself. He is confused and scared, and tells the player that the Delta Labs isn't safe and that the artifacts at Site 2 have something to do with the invasion. If the player persists in speaking to him, Ryan will affirm that he doesn't want to leave the tram. Because of this, the player leaves him to unlock the Delta Labs entrance. When the player returns to the tram, it is revealed that Ryan's body has been torn apart by Trites.
- Because Ryan was left alone on the tram, the player is indirectly responsible for Ryan's death.
- He will always die, no matter how fast the player tries to go back to the monorail after opening the doors.
- It's suggested he was chased out by a Trite that had somehow gotten into the locked rear of the tram where Ryan was hiding, as the remains of his body are located outside of the tram and there is a Trite that starts attacking the player from near the location of the remains.
- The player can see Ryan earlier by standing behind the tram. He can be seen trying to hide within.
- Ryan is one of the very few characters that cannot be killed after the demon invasion.
- He does not react to any of the enemies fought around the monorail.
- T. Ryan may be related to Mark Ryan.