Dexter: "He's kind of a self-important asshole...but I like him"
Trent Casey is a character in the Season Six of DEXTER. Casey was a professor at a local college. He was an outspoken atheist professor, which caused him to be the ideal target for "The Bowls of Wrath" by the Doomsday Killer, who at the time was believed to be Professor James Gellar. Dexter attempts to warn Casey, who arrogantly responds by saying that he is always receiving worthless death threats. Later, Dexter and Travis Marshall return to the college to attempt to catch Gellar before he can kill Casey, but they are too late. His right hand was cut off, and he was drained of his blood, similar to Brian Moser's victims. When his body was moved by Vince Masuka, a large quantity of his blood fell on everyone at the scene.
Trent's blood was then used to write: Bring the false prophet to the church on the bathroom wall of Travis' apartment. His hand was also left in the sink.
With the later revelation that Geller was just Travis's split personality, it is likely that Travis trapped Dexter in the elevator and killed Casey before 'freeing' Dexter, claiming that Geller simply evaded him.