Badass: Apparently the first person to ever serve a second 5 year term on the Rippling Walls, and was good enough that the Marshals trusted him to scout further than anyone. And that's before he takes on an entire continent on his own.
Dark and Troubled Past: He went to fight to earn money for his mother, but when he returns, his mother has passed away and the money is gone.
Determinator: Monument's broken? Get the shards. Broken again? Get more.
Heroic Mime: Occasionally people react to what he says, but you never hear him. Even at the point after Rucks's narration is done, you never hear his voice, although there is always a description of what he is asking people when he shows them items.
My Greatest Failure: He belonged to the guild that designed the Calamity weapon; though he wasn't the one who set it off, he clearly feels responsibility for the Calamity. It's why he's so obsessed with fixing the Bastion to undo everything.
Retired Badass: It's heavily implied that Rucks was once a member of the Triggers, an elite squad of Caelondian riflemen.
Why Do You Keep Changing Jobs?: According to the stuff he says on weapons loadouts, in-story, and through item presentations, he appears to have had a long and storied career. He says that he used a Hammer and Army Carbine when he was a fighting man, suggesting either a Mason or a Trigger. After completing the vigil for the Masons, he says that there's only two of them left, which could only mean him and the Kid. The Triggers' sigil has him comment that they were the bravest men he knew, suggesting he spent some time among their ranks. He was also, for certain, a Mancer in the last few years before the Calamity occurred.
A man that the Kid finds in the Hanging Gardens. He is an Ura ambassador to Caelondia.
Berserk Button: He really flips out when he finds out that the Cael Mancers designed the Calamity weapon that destroyed Caelondia, killed the woman he loved, and blew the Tazal Terminals sky high. Especially notable given the fact that, before it happened, he was a voice of peace and reason for both parties.
Despair Event Horizon: Was in the midst of one when the Kid showed up, waking up on the morning of his wedding party to find a world in ruins, everyone dead, and his to-be wife's ashes scattering at his touch. According to Rucks, he was just about to "throw it all away" before the Kid showed up.
Face-Heel Turn: Goes to the Ura and convinces them to ransack the Bastion after finding out from Zia's father's journal that Rucks and the other Mancers created the Calamity cannon.
Friendly Enemy: To the Kid and Zia. He doesn't want to harm them, necessarily. He just wants to see the Bastion fall, and perhaps take vengeance on Rucks.
Gentleman and a Scholar: And an important force in contributing to detente between his people and Caelondia before the Calamity.
Maligned Mixed Marriage: For all that the setting plays up the Ura-Cael tensions, this is happily averted in the backstory. Zulf and his fiance don't seem to have run into any trouble for getting married from either direction. Unfortunately...
Reasonable Authority Figure: Raised by a Cael man, he moved to Caelondia and worked, with apparent success, to promote a peace based on mutual respect and coexistence. The Calamity breaks him and the awful truth about it and the futility of his past work breaks him again.
A young Ura girl the Kid encounters in Prosper Bluff.
All the Other Reindeer: She is an Ura raised in Caelondia who knows next to nothing about her cultural heritage. It still wasn't enough to keep people from messing with her.
Break the Cutie: Her would-be boyfriend sold her and her father out to the authorities, saying they were Ura spies. This after more or less insulting her father indirectly and planting the idea of escaping to the Terminals in her head which led to her arrest in the first place.