The other zombie is in fact Ayumu from the Future.He has white hair, yellowish-brown eyes, cooks. Seem like a jaded version of Ayumu.
Ayumu is only convinced that Eu has a dere side because he is under her spell.She is a necromancer and raised him from the dead as a zombie to do her bidding. He is mentally bound to be loyal to her, no matter what. To rationalize his behavior he is convincing himself that she is secretly a really nice person. Maybe the delusions he has about her acting cute are even what he actually sees when she shows him notes with her commands.
The 'Serial Killer' Ayumu is looking for is the Vampire Ninja in the OP.She wields the same kind of katana that Ayumu was impaled with, plus she is clearly shown fighting him in the graveyard in the trailer to the series.
Ayumu is actually KeiichiHe looks exactly like Keiichi, gets dressed in girls' clothes, and dies a lot. It would not be surprising if Eucliwood Hellscythe and Hanyuu are good friends.
Haruna is a part of the group of girls in the Freeze-Frame Bonus portion of the OP
Haruna is Cute and Psycho
Kyouko will come back, possibly as another undead.She is featured in the Freeze-Frame Bonus, so either there is something very important about her suicide...or she's not gone for good.
Kyouko is an Eldritch Abomination in its true form.The end of episode 4 has Kyoko kills herself. She is killed by... something pure evil... that took her body. It possesses great power and overpower the protagonists and even Megalo.
Kore wa Zombie Desu Ka? Is an AU of To Aru Majutsu no IndexThink about it: Both protagonists are ordinary guys who gain an incredible power, and an incredible harem. They both fist fight, although one can cross-dress/transform into a mas(h)ou shoujo. The female cast members are similar, with Haruna and Misaka looking alike and being the tsunderes, Sera and Kanzaki being tall, dark and pretty, and with Index and Eu looking somewhat similar, and both being in a predicament where they cannot use their magic. Both series have copious amounts of fanservice.