In the anime, Reiji and Nanami were first introduced in the second season when they performed 'Poison Kiss' for STARISH, although their first real interaction began on the third season. Haruka was requested to compose a new song for Reiji. Since one of Haruka's requirements for composing a song is to get to know the person she was composing for, she had to spend the day with Reiji, much to Tokiya's and Ittoki's annoyance.
Reiji shows great fondness for Haruka, teasing her and complimenting her whenever he can. He admires her spirit and among QUARTET NIGHT, he was the first of the group who was eager to get close to her. Nanami in turn, respects Reiji due to his dedication to being an idol despite his busy schedule.
It is quite evident that Reiji may have strong feelings for her that he hides through joking around. He had once offered her to 'join the world of adults together' (implying mature themes), then waving it away as a joke. He had also confessed (with a serious face) that he loved her, only to take it back with a laugh. It may be because he had to hold back since the rule that 'idols are not allowed to fall in love' is still in place, and that saying it out loud and taking it back was the only way he could convey his feelings.
Among QUARTET NIGHT, he had been the most vocal about stealing Haruka from STARISH as their primary composer, since it was through Haruka's efforts that QUARTET NIGHT came together as a group. He had lightly threatened STARISH that they would take her away if they did not take their rivalry seriously.
In Season 4 Episode 1, after QUARTET NIGHT take Haruka away from HEAVENS, he says that "he can't lie to his heart anymore." He is then shocked when STARISH pulls her away from QUARTET NIGHT.
Although Otoya and Tokiya could get exasperated with Reiji's bubbly and happy attitude, they usually do not mind the way he acts since they respect the way Reiji could handle his professional life. They admire the way he could easily capture everyone's attention due to his easy-going nature. Tokiya describes him as a 'difficult' person and Otoya had commented that he was amazing. They look up to Reiji as their senior, and Reiji makes sure to cover for them whenever they make mistakes during work.
QUARTET NIGHTReiji and the rest of QUARTET NIGHT share a complicated relationship due to the fact that he was the only person who was willing to make the group work at first. Professionally, Reiji has no trouble working with the rest as he can easily be serious when it comes to idol work.
He likes to tease his fellow members and has nicknames for all of them; Ai is 'Ai-Ai', Ranmaru is 'Ran-Ran', and Camus is 'Myu-chan'. Despite the fact they find it annoying, they still answer whenever he calls them by their nicknames.
Reiji is the clown of the group and is quite childish around them even though he is the oldest member in terms of age and has the most experience at being an idol. A running gag is Reiji trying to encourage the other QUARTET NIGHT members to do something, and them shutting him down instantly, making him sulk.
He is also the most talkative, making Ai, Camus, and Ranmaru annoyed due to his cheerful ramblings. Still, the others value him as a member of the group, although they do so secretly. An example would this would be the time Reiji was about to go overseas and told them not to be lonely without him. Ai retorted that he would not, Camus told him to shut up, and Ranmaru told him to just go, making Reiji pout. Still, after a few hours without Reiji, Ai commented that it was too quiet for their liking.