El Capitan rapidly climbs the importance latter as the trilogy goes on: in the beginning, he's just the head of the OSR. Then, he's a member of the good guys. By the end of the trilogy, he might just be the most beloved—and most important—character. He's hard-shelled with a soft interior, he's filled with guilt but also love, and he repents all of his sins near the end of Burn. El Capitan is a changed man. But most importantly, he takes on the role of the quintessential, boundless brother.
For the most part, we get to see El Capitan transform from a bully to his brother Helmud to Helmud's best friend. Which is kind of weird seeing that him and Helmud are in a way the same person (both literally and figuratively). Before he started to change, El Capitan pretty much hated Helmud:
El Capitan used to think he understood his brother because he thought his brother was a moron, a grotesque puppet that sat on his back, forever. (3.28)
Yeah, that's not too loving. El Capitan was a bully. But really, he was a bully to himself:
It used to be that beating his brother made him feel a little more alive. He doesn't know how or why. Maybe because it was the closest thing to beating himself. (36.7)
See, El Capitan doesn't just learn to be a brother to Helmud—he learns to be a brother to himself. When he would beat Helmud, he would also be beating himself. When he hated Helmud, he hated himself.
Yet, even when the third book starts, El Capitan is still a bit sour when it comes to having Helmud on his back for eternity. Check out what he thinks when he sees Bradwell's wings:
El Capitan feels for him. He knows what it's like to haul something around on your back forever. Still, Bradwell's got it easier than El Capitan, right? At least his birds don't talk back. (8.5)
But watch how his annoyance at Helmud turns to fierce protectiveness the minute that Bradwell starts to menace him:
"Don't ever shove Helmud around […] I'll protect him with every drop of blood in my body. You got that?" (27.39)
In essence, El Capitan is saying two things:
Yep, brotherly love defines El Capitan's role in this book. He's a brother to the day he dies… even to his brothers from other mothers. He tells Bradwell:
"I'm going because I'm not letting you go alone. We're like brothers." (60.19)
Sniffle. We wish we had a brother like El Capitan.
All this bromance can cloud our memory of who El Capitan used to be: a really evil man. El Capitan was an elite killer, a semi-psychopathic murderer. But dang if he hasn't seen the error of his ways.
Turning over a new leaf doesn't quite cut it, though. El Capitan feels the need to repent. He worries that, without repentance, the weight of his sins will follow him forever.
Thankfully, he's able to be forgiven near the end of the book.
But here, now, with Helmud, El Capitan is asking for forgiveness from Saint Wi or God or whatever force might exist beyond them […] And then he feels it—something breaking open in his chest. And being lifted out. (43.93)
With this metaphorical weight being lifted off of his shoulders, El Capitan is the man we've always wanted to see him become. He's not the OSR leader anymore—he's Walden Croll, loving brother, loving son, and loving friend. Just because he says, "I always doubt people," and "I've survived by not believing in other human beings," that doesn't make him a bad guy. (13.32) In fact, it's just another way of being safe.
Without the burden of being labeled an evil dude anymore, we can look at El Capitan as the man we know him as — the unwavering companion. In fact, his character is best summed up in what he says to Pressia in Ireland's fog:
"If you were the person standing there with me," he says, "I'd always, always stay." (11.140)
El Capitan—sorry, Walden Croll—is like a pit bull. He's a little terrifying to look at (and he can be trained to be ferocious), but he's ultimately a loyal, tenderhearted friend.