Red Baron: By the end, he's known as The Warrior, for killing The Beast of London.
Stranger in a Familiar Land: At the end of the book, when Richard finally managed to return back to the London Above, he finds it hard to fit in to a normal life again. Being unable to stand this, he manages to find a way back.
Unfazed Everyman: After a day or so, he stops protesting impossible things and just starts accepting them, though he still doesn't really understand.
Unperson: After he had helped Door, no one could remember him. They also could barely notice him, unless Richard really tried to gain their attention. And even then it's really minimal.
Played by: Laura Fraser (Television), Natalie Dormer (Radio)
Aristocrats Are Evil: He's certainly not a villainous character, but he's "a little dodgy in the same way the ocean is a little wet".
Back from the Dead: About halfway through, he's tortured to death by Croup and Vandamar, but by the end he's been brought back thanks to his Soul Jar.
Chessmaster Sidekick: Casually snags what turns out to be an important trinket from Portico's study; it's a totem of the Beast Islington gave him, pulls an impressive Thanatos Gambit on Croup and Vandamar, and is generally awesome.
Wicked Cultured: He waxes eloquent about a piece of Tang dynasty china, about how it brings joy and beauty into the world, about how it's an exemplar of superb craftsmanship. Then he messily devours it.
Played by: Clive Russell (Television), David Schofield (Radio)
Psychopathic Man Child: At the end, it seems childishly confused when Door accuses it of killing her father, replying uncertainly "I didn't kill your father. I had him killed." Its temper tantrums also fit this trope.
Your Heart's Desire: It agrees to help Door find who killed her parents, and get Richard home. And get Hunter that spear. And it does tell Door who killed her parents, and why. As for Richard, finding the way home was kind of incidental to its own goals, so, technically...