I'm afraid I haven't fully explained the man in the big fat suit of armor.
The man in the big fat suit of armor is a legendary Steam Mage, nicknamed "Potbelly" by his peers for his unusual suit of custom hydraulic armor, which has a huge belly, and resembles a potbelly stove. He has fire going in his stomach area which boils water and generates steam to power his mechanical suit and that big claw on his back, and also that phone that Cogs was talking on earlier.
This comic takes place near the peak of Potbelly's career. He is known for his brutal tactics in battle, the one where he shoves a guy into his boiler and burns him alive being the most infamous. Potbelly is considered a hero all over London, which eventually leads to him marrying a trophy wife and fathering two sons.
In the game's timeline, you meet Potbelly as an NPC at a time when he's been forced to take things down a notch. His family needs him, so he doesn't go into battle very often so as not to endanger himself. Mostly he stays off to the sidelines, operating machinery, most often the stealth trains of London, which provide the safest way around the surface.
Meanwhile his sons hope to follow in his steps, and have nicknamed themselves "Barrelchest" and "Hunchback," constructing their own custom suits of armor to fit their images. Potbelly thinks that they are ridiculous for setting their centers of gravity too high.
Steam Mages are some of the most esteemed and educated individuals in Trueborn society. It takes them years of training to learn their craft, but the outcome is worth it. They are able to channel the radiation inside them into steam, making it far more potent as a mechanical driving force than it would be otherwise, and allowing them to animate massive machines with far less fuel than would ordinarily be required. Potbelly's armor, for instance, is impossible by real-life physics, and a non-Steam Mage wouldn't be able to move it, but Potbelly, being a skilled Steam Mage, is able to operate it without any discomfort whatsoever.