It is yet unknown how they met or formed their team, but they are close to each other, calling each other by their first name. Mai likes to tease him by calling him useless for having high knowledge and no stamina or strength for battle, but she thinks that he can be good at fighting sometimes. Haruhiko looks up to Mai for her amazing powers and calls her "Mai-senpai." His coordination with Mai also appears to be quite good. Mai trusts and cares for Haruhiko very much and very much is the same for Haruhiko. Mai has feelings for Haruhiko.
Haruhiko finds Mai attractive and has a preference for big breasted girls. When it comes to Mai's safety and well being, Haruhiko becomes very serious and decisive. In Episode 11, Haruhiko becomes enraged upon seeing Mai hurt from a sandbox phantom, and proceeds to destroy it without consideration. In Episode 13, Before Enigma could steal Mai's powers, Haruhiko rushes in to defend Mai, despite having his own powers stolen already.
When Mai takes care of Haruhiko as a child in Episode 11, she gains a better understanding of Haruhiko's past and relates to his lonely childhood, taking care of him and being the motherly figure that he's always longed for. Mai also tends to get jealous when other girls get too near him. However, they deeply care for each other and share a close bond. Their similar childhood often brings them closer together. They both shared the same perspective on lifestyle, and both believed that living alone was always the better alternative. Over time through their experiences with their friends and each other, they have grown to appreciate their relationship and relationship with others. Haruhiko comes to understand that Mai is just like him and is often subjected to loneliness. The OVA hints that Haruhiko has developed feelings for Mai as he almost attempts to hold her hand.
He always finds himself in embarrassing situations with her. When he met her for the first time, he accidentally saw her panties, and there is a running gag in the series that Reina throws Haruhiko when he does or accidentally does something embarrassing to her. Reina got impressed by his knowledge though she also soon starts calling it useless knowledge. At first they called each other by last names but later they started calling each other by first names.
Haruhiko cares for Reina, like when he tried to help her "break" free from her illusions created by some phantoms, saying that they should head back together and that she has a place she belongs to. Reina also thought that she was a little distant to Haruhiko, Mai and Ruru and said that she wanted to be closer to them, too. There are also hints that Reina is attracted to Haruhiko, and Haruhiko is sometimes shown to blush in front of Reina. Haruhiko sees Reina as a really kind girl and cares for her well-being. Overall, they are close friends and share a good bond.
Haruhiko, unlike everyone else wasn't offended by Koito's "rude" behavior and instead found her quite interesting, not in a very special way though. Haruhiko, along with Reina & Mai helped Koito with fighting some phantoms, even though Koito didn't want it. Even though he helped her, Koito only complimented Mai and Reina's ability and skills, much to Haruhiko's dismay.
Haruhiko was also eager to help her and cares for her, seen in Episode 5. He also started calling Koito by her first name, which she didn't like but she later accepts it as a positive one. Since Haruhiko has only seen a isolated and cold side of hers, he became very shocked when she thanked him for rescuing her, and apologized to him and blushed in front of him. After these events, Koito became friendlier and joined Haruhiko's club, so it is thanks to Haruhiko Koito became more social, and Koito claims that he is really nice to her. Later on, she also starts calling him by his first name and there are hints that Koito is attracted to Haruhiko.
Ruru is almost like Haruhiko's partner, as she is a small phantom and is always shown to be sitting upon Haruhiko's head or shoulders or flying around him. According to Ruru, they met each other when Haruhiko was in middle school. Since Haruhiko lives alone, he is happy to haave Ruru by his side since he won't have to feel alone.
They are almost inseparable and are always shown to be together. Haruhiko finds her cheerful personality annoying most of the time but doesn't reject her to be around him all the time, though he sometimes "slaps" her when Ruru says embarrassing and weird things. Ruru also seem to take over Haruhiko's leadership sometimes, which Haruhiko doesn't like.
In episode 10, it is revealed how much Haruhiko actually cares for Ruru. Ruru wants Haruhiko's attention often and she is usually seen comforting Haruhiko. When Haruhiko thought that she had died, he became very sad and said that he didn't want Ruru to leave them. When she came back, Haruhiko became relieved and a little annoyed at the same time.
However, even though their like-hate relationship, they still care for each other and are on good terms.
Haruhiko sees Kurumi as a younger-sister and Kurumi calls him "Haruhiko-oniisan". Haruhiko seems to impressed by her abilities and he cares for her. While spending more time with each other, they grow to be quite close and Kurumi joins Haruhiko's club. There are small hints that Kurumi is attracted to Haruhiko, as she claims that he is the one who gives her courage.
Shosuke is Haruhiko's friend, and they are very close. They eat lunch together sometimes and they can talk about various topics with each other. They are seen to encourage each other, but Shosuke seems to be slightly jealous of Haruhiko since his team only consists of pretty girls.