Zaō has been shown to be close friends with Io, which has been established since episode one. Io has exhibited behavior that is protective of his friend, such as in episode six when he does not partake in the battle until he sees that Zaō has been endangered. This is expanded on in episode seven, in which Zaō is accused of "liking" him during the incident of finding out who used Io's toothbrush (fortified by the fact that he claims that "women are life," despite never actually being seen with one.) Though in Season 2/2nd TV series we see Ryuu get lots of chocolate from girls. Also, in flashback, he is said to have a girlfriend and on New Year's Eve was planning to go out with girls.
Io and Ryuu's friendshio, however, is punctuated with some romantic attitudes. 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.
The two don't seem to get along as shown when Akoya glared at Ryuu. The reason for this is because when they were first years Ryuu and Akoya were both tied for third place in the prettiest boys competition. They haven't gotten along since.