Io and Zao are very close friends. It also appears that Io is protective of Zao to a degree, as shown in episode 6 when he only joined the battle after seeing Zao in trouble. Their friendship may be also a bit romantic. In Season 2/2nd TV series and entire episode on how they met. Initially, Ryuu and Io couldn't seemingly stand one another. Io was put off at Ryuu's dating and thought it to be vulgar. Io comments that dates are poor investments. Ryuu comments that it pretty vulgar to feel that only money is valuable. When they join the Earth Defense Club both of them seem to not want the other around but then once they go to Kurotama bath they seem to get along. This is because Yumoto accidentally drops water on them and Gora apologises and tries to get them to enjoy the bath. The viewers later learn that Yumoto burned their school jackets when ironing them and he attached matching love emblems on them and though Io and Ryuu seemed to hate them they seem to love the fact they shared this connection. Their fight broke out because Ryuu was saying to his schoolmates that Io is cool and Io disagreed, now stating, his dating stimulates the economy, so he is cool. When En stated that they may like each other they both yelled that "yes" but frustrated that the other never admits it. In Episode 9 Ryuu gives Io chocolates on Valentines Day thou Io acts indifferent he blushes as he accepts the offering. Despite their seeming differences Ryuu and Io get along pretty well and it is hinted it has some romance in it.