Breast Expansion: On the meta level the author admits that the characters breast size varies by shot - which he jokingly refers to as "convenient cleavage syndrome" - and that the average cup size for his characters has gone up over the course of the comic. He lampshaded this a few times in universe, with special mention going to Serena, whose breasts apparently grew right before her debut.
Bishōnen: Blue's convention friend, literally named "Mr. Bishie."
Element Number Five: Played with. The primary forms of magic in the world seem to be Earth, Water, Power (ie, Energy) and Fire, and a Fifth Elemental Power effectively completes the set. The oddity of this is that the fifth power is actually a pair of elements: Life and Death, each of which have their own user. Despite this, they are counted as a single power for most intents and purposes. Also present are demonic and celestial magic, both of which are implied to be weaker than elemental magic.
Faux Yay: The Demoness loves to invoke this, such that she and Blue compete for the title of The Tease.
Fantastic Racism: Played with. While Chimeras aren't completly hated, people do tend to look down on them. This has been slowly getting better according to the backstory.
The subject is showing signs of returning in the current arc where Kate and Phil plan their wedding. Apparently, five years after "The Weirding" (the event that created the first Chimeras), Kate and Phil's home state declared that wedding officialors had "the right to refuse" legalizing a wedding if they so wish. As Phil himself points out, the chances of this being a coincidence are small.
Orichalcum: Used by Shiden's creations as a Power Source.
Our Demons Are Different: Though quite possibly a succubus, the Demoness has yet to be shown doing anything succubus-like outside of teasing everyone with her sex appeal. Penpen prefers spending time engaging in a gelatin fetish over acting like a Demon Prince; his cousin (who has yet to be named) hasn't seen much character development yet, but so far he's not evil. Axel's Demon is the only one reminiscent of a demon (both in appearance and personality).
Otaku: Phil, to the point that he knows manga occurances by panel
Quiz Host: Err- OK! How about quoting page 52 from Volume 6 of XX Xholic- last panel?
Phil: Watanuki tells Yuko- "No dessert for you!" and she says, "You're joking."
Replacement Goldfish: Averted. Cain goes to a lot of trouble to bring his real sister back from the dead.
Robot Girl: Miyo is only the first of these, built by Phil's frenemy Shiden. Past a certain point, he has entire facilities full of them, performing a wide array of jobs from assistants to bodyguards. They also play with the notion of Ridiculously Human Robots, as they do have emotions and desires separate from their programming (which Shiden did allow for), as well as needing sleep and food (or reacharging).
The Load, The Fool: Phil, again—he's just a regular human, with no real abilities aside from a really sunny disposition and a kind heart. Interestingly, his ability to recover from what would be considered intense physical trauma actually goes from a comedic device to an actual ability - as the Null, similar to Asuna, he naturally nullifies magic and magic artifacts, though his area of effect extends a bit beyond his body. But unlike the previous Null, for less explained reasons, he is also Nigh Invulnerable, capable of surviving hellfire, painful falls, and hits from training machines made for robots with no trouble.
Well-Intentioned Extremist: Cortex will do anything to stop the Magi, including attacking a diner full of civilians just to get one girl and, more recently, trying to steal several priceless artifacts. Of course, the latter turns out to have been a Kansas City Shuffle in order to gain access to an ancient magical seal.
What the Hell, Hero?: Axel is...less than pleased when he learns how the group has made friends with Lia.
It gets worse. All of the heirs to the power of the Magus of Power are like that. They are raised like that so that they become puppets to the power, which in turn makes them puppets to the rest of the family, because they would do anything to prove themselves.