Haruhiko first met Reina when she ate a Phantom and he wanted her on his team. And some time after that he accidentally saw her panties which made Reina throw him, and there is a running gag in the series that Reina throws Haruhiko when he does or accidentally does something embarrassing to her, like holding her hand or hugging her.
Haruhiko asked Reina to join their club and Reina later agrees sometime later after declining the offer. Reina thought that Haruhiko's ability was cool but starts she also soon starts calling it his knowledge useless like Mai. At first they called each other by last names but later on they started calling each other by first names. Haruhiko sees Reina as really kind girl and cares for her. He wanted to help her from breaking free from her illusions created by some Phantoms. He told her that she had a place she belonged to, which deeply touched Reina since her parents are quite strict.
After this there has been small hints that Reina could be possibly interested in Haruhiko. Haruhiko is shown to be concerned about Reina's well being when something dangerous happens and sometimes even blush in front of her. However, they are on good terms and share a close bond.
Reina and Mai are very close friends and became friends very quickly. Reina thinks that Mai reminds her of her older sister who left home and therefore calls her "Mai-oneesama". They deeply care and trust each other and almost have like a sister-bond. They can freely talk with each other about various topics and Mai often tells Reina to be careful when she eats phantoms since it can be dangerous. Despite being very close and good friends, Mai is shown to be quite jealous when Reina and Haruhiko gets too close.
At first they were only but acquaintances, with both of them not thinking anything special about each other. However, after the series progresses they start spending more time with each other since they are in same club, and also starts calling each other by first names. Koito isn't bothered by Reina's big appetite and thinks that it's her reward for her hard work, which makes Reina happy to hear.
Reina and Ruru are both on good terms and care for each other. Ruru feels comfortable being around Reina and as well Reina. Sometimes, Reina is seen to feed Ruru.
While they aren't particularly that close, Reina still cares for her and her well-being since she is a child. Kurumi also looks up to Reina as an older-sister.