The Corruption: The more he uses the keys, the closer he gets to becoming a Denzien. Or so he thinks at first, but it turns out humans can't become denizens so he's changing into something else.
Expecting Someone Taller: Thanks to a 'Thesaurus' denizen he rescued from the pit in Book 2. From book 3 onwards, everyone shows disbelief at who he says he is just because he's not 10 feet tall, has a beard and isn't accompanied by a frog/bear hybrid.
The Power of Friendship: Arthur has a habit of making friends wherever he is. It might help however that he says please to denizens, who are surprised someone higher than them actually says it. He gets a lot of extra help this way.
Scaled Up: Can transform into a massive serpent to attack.
Cool Train: Covered in spikes on the outside to stop denizens climbing on it, and is the only way to get in, or out, of the massive pit in the Secondary Realms. Also has an equally cool train station (Which Tuesday has to repair all the time because it keeps being degraded by nothing) copied from one on Earth.
Creative Sterility: Can create beautiful things, and combine existing things to make something new, but not from scratch.
The Collector: Has a tower filled with treasures, including The Mariner.
We Hardly Knew Ye: Physically appears very rarely, though is behind most of the evil throughout the books.
Affably Evil: He offers to let Arthur go free if he relinquishes his Keys to him, which will enable him to stop the flow of Nothing, and will not get involved in further intrusions on Earth. However, he does so whilst threatening to have Arthurs eyes removed...