Kimi no Unaji ni Kanpai!

Kanpai!, or Kimi no Unaji ni Kanpai! (キミのうなじに乾杯!, lit. "A Toast to Your Nape!"), is a Japanese manga by Maki Murakami. Originally published by Sony Magazines, its two volumes were published in English by Tokyopop.

Plot
What with humans killing, exorcising, and otherwise getting rid of them all the time, monsters and demons have become endangered species. Someone has to look out for them, and that's what Yamada Shintarou is training to do- he's going to become a monster guardian. Armed with a two-by-four and a talking rabbit, he goes out to fight the demon hunters, hoping to defeat enough of them to get his monster guardian license. It is while he is doing this that he meets Nao, a normal (if unusually pretty) high school girl, and instantly falls in love... with her neck. Now Yamada has a new mission in life: to protect Nao, and keep anyone- monster, ghost, demon hunter, or otherwise- from getting in the way of his view of her nape.