Rei Ryugazuki

"'So what you're saying is, if they disappeared no one would notice?' - Rei Ryuugazuki, episode 10"

Although he is first seen in episode 7, Rei doesn't really make his debut as a main character until episode 8. After two episodes of pressure from Nagisa, Rei finally joins the swim team despite not knowing how to swim. He appears to be a serial killer in the making and although he's very intelligent, people seem to be generally creeped out by him.

Personality
Rei is creepy not only because of his obsession with crime but also because of the sound effects that play when he speaks. His track teammates seemed disturbed by him and according to Haru he spends his entire scrap booking class cutting the eyes out of pictures. Despite his desire to meet Nagisa's teammates being because no one would miss them if they disappeared, after joining the team he seems to genuinely care about their well-being. He also sees right through Nagisa's insistence that he doesn't like either Makoto or Harucum.

History
The turning point in Rei's life was when his underwear were stole while on a 5th grade field trip. He is convinced that a girl named "Miko" stole them because earlier that day she called him a "fart smeller." Although this appears to just be a childish insult, to Rei it was much more. He vowed that when he got older people like Miko would be taught lessons.

Trivia

 * In episode 12 he has "Property of Nagisa" written on his lower back which appears to be a tattoo however isn't seen again.
 * Rei and Nagisa appear to be in a relationship. At the very least they have feelings for each other. The only reason Rei does favors is because of necessity to maintain his cover, and also because of these feelings for Nagisa.
 * He is at the very, very top of the criminal hierarchy; this is shown with his extensive knowledge of criminal shenanigans and knowledge of Nagisa, someone lower down than him, including all aliases and criminal history. He is disguised as an academically-gifted high school swimmer as his cover-up; a disguise so well-planned that Easter Dave and Miho do not recognize him as The criminal overlord (Miho considers him and Nagisa to be "small-timers") and Haru and the others do not suspect a thing.