Zastin

Lala's personal bodyguard and the best swordsman on Deviluke. Initially, Zastin disapproves of Rito, deeming him weak and subsequently seeks to test his strength. However, once Rito exclaims his opinion of marriage in self-defense, Zastin is swayed by his words (though not in the way Rito intends) and summarily approves of him, believing he truly understands Lala's feelings. He bares some resemblence to Creed Diskenth from the manga series Black Cat another series by Kentaro Yabuki.

Personality
He is currently working as the assistant chief to Saibai Yuuki the funds he gets for his duties are handed to Lala for her use. Zastin has shown a keen interest in becoming a mangaka, professionally. He has two assistants that work under him, simply known as Maul and Smutts. Zastin's tail is thin and very scorpion-like in appearance.

Despite his skill with the sword, Zastin has a horrible sense of direction, resulting in him getting lost (one of the running gags in To Love-Ru), Rito states he can be a bit dull, such as when he brags about his title as assistant chief of a manga artist to his enemy, being easily persuaded and constantly working with Rito's dad in drawing manga when it's supposed to be his job as a bodyguard. He also seems to have extremely bad luck, where he keeps getting run over by trains and cars.

Gid Lucione Deviluke
Out of everyone in the series, Zastin looks up to and respects Gid the most as the King of the planet Deviluke and Emperor of the entire Milky Way Galaxy. He is one of the bodyguards who protect the king's family members and would get mad at anyone if they disrespect any of them. Due to having the same last name, it's possible that he is related to him in a way, but this has not been confirmed.

Lala Satalin Deviluke
As the first born and heir to the Planet Deviluke, Zastin shows respect to Lala. Despite his loyalty to her, he often seems to have a miserable time serving her. He encouraged and urged her to behave more properly like a princess, and was forced to follow her when she ran away from home. And many of the troubles caused by Lala would end up giving him trouble as well. It was once said that when Lala was younger, Zastin was the guinea pig for her inventions and tried to run away, only to be tracked down and caught by her robotic search dog, Sniff Sniff Trace-kun.

Nana Aster Deviluke and Momo Velia Deviluke
As daughters of the King of Deviluke, Zastin expects them to follow orders as Princesses of Deviluke, but instead they follow their sister to Earth to escape from their studies, which also causes him trouble even after he failed to bring them back.

Rito Yuuki
At first he didn't accept Rito marrying Lala and becoming the heir to the throne and the future King of Deviluke and Emperor to the Galaxy, stating it's not a position for weaklings and battles him so he would step down. However, after hearing Rito's speach about how marriage would only work if two people love each other, he mistook this as a right that he is the only one suited for Lala. He then accepts Rito's right to marry her, which Rito did not originally intend.

Afterwards Zastin started showing respect to Rito and even called him with the polite honorific Rito-Dono. He occasionally would help Rito out when he is stuck in a difficult situation involving something from space and would go to great lengths to protect him (even though he often fails in the process).

Saki Tenjouin
Saki became smitten for Zastin ever since he helped lift her car out of a hole in the road. While Saki made a few attempts to get his attention, most of those times, one mishap after another would throw her into an embarrassing situation in front of him, such as revealing her panties to him, which he tries to politely deny seeing, which does little to convince or console her.

While Zastin is formal and polite towards her as he is to anyone else, it is unknown what he feels towards her, but there is little evidence that he has romantic feelings towards her. Despite being in consideration to be included into Momo's harem plan, Saki appears to be the only female character with affections for another male character besides Rito, which may cement her as one of few girls who is genuinely not a love interest for Rito.