Rei is described as having blue hair, purple eyes and is wearing red glasses.
He urges Azusa to join the swim team because he believes she would look beautiful.
He later has ice cream with Nagisa, where Akira shows up and confesses her love to the shota. After she has left she calls Nagisa, and Rei joins them after Kimmy has already showed up.
Rei thinks Azusa would look beautiful while swimming and Makoto believes the two would surely get along as they're both new members.
Rei and Nagisa are confirmed to have been dating even before Azusa randferred to Iwatobi. Whether and how much Rei knows about Nagisa's involvement with the yakuza is unknown.
- From all the characters from the Free! canon he appears the least. If you exclude Nitori, that is.