Tsukasa Futaba is one of Subaru's classmates, who is always seen with a Wave Scanner in his hand (which contains Gemini). He was involved in a vehicle accident in which a truck hit him as he was crossing the street. He was left to dead when 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.