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Leader of the Deviluke Royal Guards and personal bodyguard of Lala.
Zastin is first seen in the opening when he is attempting to chase Lala and bring her back home. Needless to say, he fails (else the entire manga or anime would never happen).
Zastin pretty much crashes in through Rito's window in both manga and anime (although he has two goons along with him in the anime) and either dule's Rito until he is hit by a train (manga) or until Lala sucks him up with a giant vaccum (anime).
A common trait between both anime Zastin and manga Zastin is the total helplessness of him. He has no sense of direction, so he is always getting lost. And his luck is absolutly terrible, hence him getting struck numerous times by a train or a car. He also tends to fall down manholes and get bitten by dogs. In the manga he also has the misfortune of being possesed by a perverted ghost and teleported to a deserted planet and left behind.
Somthing that is unique to the manga Zastin is his shift in jobs. While visiting Rito's dad, Lala uses one of her inventions to help the assistants to draw faster, the catch of the device is that it completely burned them out, causing them to quit the very next day. So, to replace them, Lala orders Zastin and his men to help Rito's father with the manga. Zastin (manga) really starts to actually like it and is berated by Gid Luciano Deviluke for becoming distracted from seving Lala.
The only possible spot of good fortune that he might have is the romantic attention of Saki Tenjoinin. Although he is never shown to return her affection Saki falls head over heels for him and always tends to embarrass herself in front of him. Zastin also tends to react to these ecchi moments with much more cool than Rito would. Always saying only, "I saw nothing."