Awesome Music: A textual version is Arthur's song in Grim Tuesday, which won him the Second Key. And it was just a song he made on a xylophone when he was a kid.
Complete Monster: The antagonists are largely cursed, have sympathetic reasons or both, but Superior Saturday is an out-and-out monster. Her method of disciplining subordinates is to turn them inside out and turn their blood into glass, essentially making them an organ jar. Since her subordinates are immortal, this is a Fate Worse Than Death for however long it's in effect. She's practically the Big Bad herself, being behind most schemes since the beginning of the series, including releasing a mind control virus, attempting to nuke a town, throwing a man into Nothing to be dissolved and blaming his brother, and more. She wants to be the woman in charge. She wants the Incomparable Gardens. She should have been favored during the creation of the Universe, not Sunday.
Designated Villain: For someone whose sin is Pride, Lord Sunday's sole shown (rather than exposited) motivation is to be left alone and for the universe to continue existing. Despite the fact that his key's primary power is to override the other keys, he leaves the heroes alone and doesn't ever step in to hold him down until Arthur is on the verge of destroying the entire universe. His section of the house is well-maintained with happy, productive residents and he refrains from doing any more than harming Arthur and bluffing him a bit when he needs to obtain the keys, even though most people would probably stoop to a bit of harm solely for spite's sake.
He could been a little less cryptic and just have explained to Arthur exactly what the situation was, and could have avoided a lot of the bad stuff happening in the preceding books simply by taking an interest rather than considering it beneath him to get involved. Also, the way he parts the clouds on the underside of the Gardens purely to taunt Saturday absolutely screams pride.
Neutral Evil: Lord Sunday. He doesn't have any grand scheme, any ambition, all he wants is to be left alone in his Gardens, despite the severe repurcussions to the rest of reality.
What Do You Mean, It's Not Symbolic?: Everywhere. The series itself is named for a line from the gospels, and it just gets deeper from there. The Incomparable Gardens are a fairly obvious reference to the Garden of Eden for a start, and given that Lord Sunday's sin is Pride, he could be interpreted as the equivalent to Lucifer. Saturday's Tower, built with the intent of reaching the Gardens, calls to mind the Tower of Babel. The Incomparable Gardens are also hosted on "Drasils", which is suspiciously similiar to the word "Yggdrasil", the World Tree of Norse mythology. Not to mention the Seven Deadly Sins and Seven Virtues from the Christian faith.