Futaba, Tsukasa

双葉ツカサ

Tsukasa Futaba is one of Subaru's classmates, always seen holding a Wave Scanner (which contains Gemini). He was involved in a vehicle accident where a truck hit him while he was crossing the street. He was left for dead until Gemini saved him and offered him power. He accepted this power and became possessed by Gemini.
Tsukasa can perform Denpa-Henkan with Gemini to become Gemini Spark. In this form, he takes on two bodies - a white and a black version. They both seem to have slightly different motives. Gemini Spark is viewed as the main antagonist in the anime.

Spoiler