Not much has been developed about Rees yet. What we know is that he is a loyal and well-intentioned Gatekeeper in Season 1, serving under Warner. He strives to do his best despite the pressure from Warner's aggressive and volatile nature. Rees lives in Archeston with his wife, where he is based.
Timeline
Story Beat for Rees in Season 1: Under Warner's orders, the Gatekeepers venture to the Anyth to capture a hacker who has been stealing millions in currency from a criminal network in Archeston. Rees eventually tracks down the hacker and imprisons him. Later, he has a difficult conversation with his wife, who expresses her disappointment. She doesn't see much wrong with the hacker’s actions, pointing out that he stole from criminals and used the money to enjoy life and help others, making himself feel less alone. Now, all of that is gone, and Rees is responsible for locking him away, possibly alongside those he stole from. She challenges Rees, asking, “Is that really justice?”—a question that makes him pause and reflect.
Rees defends his actions, saying, “We seized all the criminal money; we can now put it to more proper use.”
His wife counters, “What? To the oligarch? To the Heavens? And what will they do with it that they couldn’t already?”
After this exchange, she begins to see Rees differently, as if he has changed, and slowly loses interest in the man he has become.
By the end of Season 1, Rees is promoted to Standing Keeper, taking charge after Warner's disappearance or death and Rogue’s imprisonment.
In Season 2, we learn that at some point during the 10-year time jump, Rees stepped down, which explains why Lace (potentially) is now in charge.