Kanpai!(2000)

Kimi no Unaji ni Kanpai!

Yamada Shintaro is training to become a monster guardian--his job is to protect the monsters from harm. But when he meets Nao, a girl from his middle school, he suddenly falls in love...with her neckline! Shintaro will go to any lengths to prevent any disruption to her peaceful life--and preserve his choice view of her neck!

(Source: Tokyopop)