Earth's Conquest Club

Kinshiro shares an amicable bond with Zundar, Ibushi and Akoya, and is their leader, working under Zundar's command. He is shown to deeply care about his Club and his goal of world domination, as seen in Episode 6 when he watches the night sky and remembers a portion of his childhood dream with Kinugawa. When questioned by both Arima and Akoya for what he wished for upon the shooting star, he pauses and then replies that it's world domination, implying that he may or may not have wished for it all the same, but he is determined in not letting his club fall apart. He genuinely cares for his club and its "Total Conquest Project".

Ibushi Arima

Kinshiro and Ibushi were the only ones in the Student Council Body in their 2nd years, which was before Gero's arrival. In the anime, he is always seen to be beside Kinshiro. It is unclear that at what point both he and Arima became friends, but he seems to rely on Arima.

In the manga, Kinshiro and Atsushi met Arima in childhood but when they met again in high school, Atsushi remembered Arima, Kinshiro did not.

Akoya Gero

Despite a year age gap, he calls Akoya on first name basis as they are good friends. Akoya respects him, particularly for the fact that his interest lie in world domination and in creating a world of order and discipline, which happen to coincide with the interests of the of the club (as  mentioned by him), and majorly due to the fact that both Arima and Kusatsu were the only two at school who did not make fun of his last name "Gero".

Atsushi Kinugawa

In the beginning of the anime, Kinshiro was distant in his approach to Kinugawa, a direct contrast to his warm-natured personality as a child. They used to be good friends in their childhood, calling each other “Atchan” and “Kin-chan”.

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They were shown to be close, often meeting up to go star gazing, as that was Atsushi's favorite hobby. They were both good in school academics, which explains why Kinshiro may have been fond of him.  
Kinshiro and Atusushi as children

Their frienship ended when Kinugawa went for curry with En instead of coming to his house, Kinshiro believed this was Kinugawa rejecting his friendship. They stayed distant due to Kinshiro holding a grudge and Kinugawa not doing anything to try resolve it. This may have been because he did not understand what had happened in the first place.

He now knows that Kinugawa belongs to the Earth's Defense Club, which they're up against. This was discovered in Episode 11, following this discovery Kinshiro turns into "Dark Aurite."

In Episode 12, Atsushi saves him from a falling pillar by taking on a direct hit in order to protect him, asking him if he was alright in the process. He says that even though he (Kinshiro) may still not feel the same way, he (Atsushi) still considers him a friend till date, despite the distance between them.

In the end, it is shown that they do end up reconciling, following which they were seen chilling at the Kurotamayu onsen, together with the rest.

After reconciling with Atsushi at the end of season 1, he is shown to be a happier person. He sometimes gets flustered by Atsushi and even the mention of Atsushi can bring a blush to his cheeks as seen when Ibushi suggested that Kinshiro should email Atsushi "for no particular reason" after Kinshiro received numerous ominous omens. He also became overprotective of Atsushi; when Atsushi didn't reply to the email, Kinshiro's first response was to fly himself, Ibushi, and Akoya back to Binan as fast as possible with Ibushi commenting that he had never had a faster trip home.

En Yufuin

Kinshiro doesn't seem to like En all that much, or approve of "Atsushi hanging out with slovenly boys such as Yufuin", as he believes him to be the reason for his and Kinugawa's distance. 

Yumoto Hakone

Though in the anime the two don't seem to interact, their relationship seems to be friendly in both the manga, and the Otome. In the manga, Yumoto pulled Kinshiro around the entire arcade, inviting him to play the different games with him.