Realtalk though my actual headcanon about Bitters personality-wise comes a lot from a particular passage in thought-‘s fic But Only When Skilfully Tied which you should read but anyway, following an assassination attempt on Kimball:
"And let me guess," he says, still sounding profoundly bored, "You're gonna deliver him straight into the hands of the prison industrial complex, because that's apparently a thing we're still doing even though we spent like eight years fighting against that sort of Federalist bullshit. Cool. Nothing problematic there at all."
I adore this because it looks to me like a Bitters who isn’t actually apathetic at all, just deeply burned and disillusioned and, well, bitter, for very good reasons, and while he absolutely still cares and is still in the fight, he maintains a level of emotional detachment because he kind of has to. He comes across as an apathetic cynic to a lot of people but I’d wager a guess that Kimball knows him well enough by now to know that bored sarcasm is largely a coping mechanism.
See also: the “Grif isn’t truly lazy” school of Grif thought.
Anyway it’s really great characterization and that one paragraph has shaped pretty much my entire view of Bitters because it’s great and because angry young people are my lifeblood, thank you and good day.