Reika is a mysterious person seems to know about the connection between Midori and Yuusuke. She seems to be intent on keeping the two apart, trying to put Yuusuke and Futaba together instead. Reika's motives are explained near the end of the series. Her given name in Japanese means "lovely flower".

Unlike the other characters, Reika was not in the H-game of Green Green. She was created as a new character for the television series.

Source: Wikipedia