"Bitch" for Rei, which has been taken up even by her fans ("Bitch is cute~", "Nice, bitch"). Also "Roach" for her hair style.
"King of Braves" for Hirano. Also "Hirako" (平コウ)).
"Excalibar" for the crowbar Takashi uses at the end of Act 5.
Generally, "Awakening" (覚醒) (a Mobile Suit Gundam reference) whenever anyone gets their first moment of heroism/craziness.
Genius Bonus: Zeke is known as Zero in the official English dub. The Mitsubishi A6M Reisen fighter plane had the official reporting name of "Zeke" among the Allies, but it is far, far better known as the Zero.
Schedule Slip: Daisuke and Shouji's regular schedule in Dragon Age Maganize began to slip after the 20th chapter or so, monthly break followed by monthly break, eventually going for a hiatus after Chapter 29.
Series Hiatus: Chapter 29 marked the longest break the mangaka duo ever took for their series, Kadokawa Shoten accordingly put the series on hiatus, taking their time with rereleasing the first 4 volumes in full color; meanwhile Shouji Sato (artist) is working on his other series (Triage X), and still works on what made him rise to stardom in the first place.
Two years after the release of Chapter 29, Chapter 30 was published....and then the series went straight back into hiatus.
What Could Have Been: The end of Volume 6 showcased a few designs and ideas that didn't make the final series:
The original title for the series was School Zombie! KILL! KILL!
Takashi was designed to be more "hero-like".
Rei was originally designed with shorter hair and her famous hair style not present.
Hisashi was going to mill around a bit after getting zombiefied.
Saeko was going to be younger and have a different personality.
Shizuka was originally a clinical nurse, as well as older and more composed than her current character.
Saya didn't originally wear glasses and was meant to just be an overbearing character.
And finally, an unused character named Tomoe Arise, who was meant to fill the "bespectacled genius" role. Since her personality was already settled on before she was cut, the creators just gave it to Saya.