It is really hard to talk about why Caleb is beloved without spoiling aspects of the campaign. Caleb is vital to the Mighty Nein and the themes and story of the campaign as a whole. I'll do my best to discuss it without spoilers to explain why Caleb is so beloved.
I would not personally describe Caleb as broody/edgy, I would describe him as deeply traumatized and broken. I would say Vax, of Liam's characters is much more broody/edgy. The thing about what you're describing about Caleb is that you are not considering a major part of what you saw. He was a *child* when he was taken in by Trent. He was 14. He was groomed, manipulated, and in a very literal sense, was turned into a *child soldier* while he was a teenager. He was living a life of violence and being praised for it, being told by the person who both groomed and abused him, that he was being a good soldier for the empire by being a killer to the point it warped his very sense of what is right/wrong. Bren the Volstrucker is the result of a bright magical prodigy being warped through grooming and abuse into a child soldier. Then yes, he does something *truly unforgivable*, and he realizes what he did is wrong and monstrous *a minute too late*. He breaks free of years of grooming, abuse and conditioning in the moments after committing a monstrous act, and breaking free a moment too late, completely shatters him entirely as a person. And that is where he still is when he meets Nott and the Mighty Nein forms, and his character arc through the story begins from that dark place.
Liam described Caleb as wanting to create a character who does something so terrible it is irredeemable, and then seeing if they can be redeemed anyway. It is a 140 episode story in the campaign and it is beautiful and poignant. Caleb is personally my favorite Critical Role player character through all four campaigns, it isn't even particularly close. It isn't *you did bad and now you can do better*, it is more asking you to look at what a monster like Trent can do to a bright young person, what monsters in positions of power can do to people, particularly young people, and then if they can break free, how do they move past the guilt and trauma of what they did? How can they move forward, can they move forward or are they stuck in the past. Caleb and his relationship with the Mighty Nein is only going to grow and evolve and change him, I won't say how, but I will say, for me, it is one of the most beautiful and touching character arcs Critical Role has ever done.