Iori is a blue-haired college student with a fairly muscular medium build. Normally, he wears a hoodie or jacket over a T-shirt. He is often seen naked with the exception of his boxers when he becomes intoxicated.
Iori is first depicted as a relatively normal boy just entering university who wants to lead a normal and peaceful school life. However, after joining the diving club, Iori spends most of his days intoxicated, naked, and partying with the club members. He is rather perverted as well. While he is generally a loud and boisterous idiot, Iori displays a kind and caring side as well, such as when he stuck up for Aina when she was being picked on and how he defends Kohei when he is insulted for being an otaku. Despite no initial interest, Iori comes to love diving and spending time with the members of the club, whom he becomes friends with and cares about despite their rambunctious behavior and endless crazy schemes.
- Nanaka Kotegawa: Due to Iori's father being originally adopted, she and Iori are not related by blood. Unlike with his relationship with Chisa, while it was not shown how they interacted in their childhood, it can be inferred that it was at least similar to the relationship Iori had with Chisa. Currently, they have a fairly happy and pleasant relationship. Nanaka looking after Iori like a little sibling and caring about his wellbeing. While Iori cares about her in a very similar manner, despite admitting he finds Nanaka very attractive (Honestly, who wouldn't?). Despite Nanaka's fanatical love for Chisa, over the course of the series, she comes to view Iori as an acceptable boyfriend for her little sister. This, however, does lead Nanaka to become incredibly protective and watchful over Iori's love life. Refusing to let him stray away from her beloved sister, and wanting to make sure Iori protects Chisa from flirty people as much as possible.
- Chisa Kotegawa: The same as her elder sister, she and Iori are cousins unrelated by blood and were childhood friends a decade before the start of the series. Not much is known about her childhood with Iori, however, it can be inferred that they were fairly close with one another from the shot of the two running with each other along the beach shoreline that we are shown. This being said, however, their reunion as college students was anything but smooth. Their very first "meeting" was Iori getting a passing by glance at Chisa while she changed out of her diving suit after getting out of the water. Leading to the two have an awkward staredown before Chisa gave Iori a puzzled look and Iori simply continued to walk off on his way. Afterward, their first "official" reunion didn't go any better, in fact, it went far, far worse. Chisa walked into the Grandblue shop eager to meet her cousin again after over a decade apart, only to find him stripped down to his boxers and dominating his fellow Peek-a-Boo members in a drinking match. This lead Chisa to gain an immediate disgust in what her cousin had seemingly become in their adulthood. Proceeding to write the young man off like an idiot, going as far as to say "I never imagined you'd grow up to be so stupid." Presumably wanting nothing to do with him anymore. Smacking away his hand as Iori attempted to greet Chisa by placing a hand on her shoulder, and taking off the jacket she was wearing and urging her sister to burn it before slamming the door behind her as she went to her room. Much to Iori's bewilderment and sorrow at this misunderstanding. This, however, was far from the end of their interactions. Over the course of the time, Iori spent living in the Kotegawa household, he began to slowly but surely mend the rift he inadvertently created between the two cousins. Getting caught up in each other's(mostly Iori's) comedic antics, Chisa still wanting to show her cousin the wonders of the ocean she loves so much, Iori getting fully invested in said world, the two becoming "Pretend boyfriend and girlfriend", and many more experiences lead the two to somehow form a strong and caring relationship. The time spent living together leading the pair to almost become in sync with one another when performing tasks like cooking. Very much having the interactions of siblings or an old married couple, much to Chisa's chastizing and declarations that Iori's an idiot. This relationship created an almost unspoken mutual trust between the pair. Iori coming to Chisa for help with his diving practice, Chisa never hesitating to lean on her "Boyfriend" whenever someone tries to flirt with her. It can be said that Iori and Chisa have come to care about each other's wellbeings quite a bit. Iori not hesitating to put his life into danger when he thought that Chisa had fallen down a cliff in a drunken stupor, Chisa always making sure Iori is enjoying himself and being involved when they go out diving(which is sadly not very often.), and seemed genuinely concerned for her cousin's emotional state when he was left out of diving for not properly obtaining his license, and Iori even going as far as to miss his flight back home to stay behind and figure out the mysterious case of Chisa's mother's hearing. (Although this can be argued to not be his original intention as Iori himself stated he simply stayed behind to get back the Pornographic magazine he forgot.) This lead Chisa to give Iori a genuinely heartfelt and tearful thank you after Iori came to her in the middle of the night to clear up the misunderstanding involving Chisa's mother. (which had previously been tearing Chisa up on the inside.) Leading the young woman to tell her cousin "I'm so glad you're here!", the warm gratitude on Chisa's face leading Iori to blush as he was caught off guard. The case of Chisa and Iori's relationship and whether or not it could ever be romantic is hinted at and visited multiple times throughout the series. Despite the two dismissing the idea seemingly whenever it's brought up, Iori saying Chisa's like a sister to him, and Chisa simply calling him an Idiot. However, with the two seemingly becoming closer and closer the more the story continues, and subtle hints being dropped every so often about their feelings for one another, it can be stated that while they currently might not have any romantic feelings for one another, it's entirely possible that said fact can be subject to change in the future.