Akio is Wataru's friend from home in Tokyo. When he, Minai, and Wataru were taking their entrance exams, they had a plan to eventually become the elite men in society. Unfortunately, that plan was cut short when Wataru had failed. While Wataru is on Promised Island living with his sisters, they keep in touch with each other through e-mail. There's even one episode where he just can't resist in playing a little friendly joke to Wataru involving an e-mail pen pal, kind of to remind him that he has a friend back home.
As far as the girls' reaction to him, he is well received as Wataru's friend when he comes to visit, though he seems to have a certain other purpose for coming to the island.
Akio was a character exclusive to the anime and only appears in Season 1 of the series