Hiroki doesn't appear in the anime until Episode 7 of Season 1. The only difference between Hiroki and Kakeru, is that Hiroki is stronger than Kakeru. It is already proven in Episode 7 of Season 1 and Episode 34 of Season 2. Hiroki has a huge crush on Natsumi, and he is always willing to do anything for her, just to win his love. 

An evil monkey, Specter, has escaped a circus, using a mind-controlling device which allows him to become smarter and have access to controlling the monkeys. Kakeru is called over by a professor in order to capture all the monkeys, including the one's who's controlling them: Specter.
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(Source: Anime News Network)


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