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.
Gemini Spark kills all of the FMs (save Cancer and Harp) in order to collect the Negative Energy that FMs are made out of. This Negative Energy is manifested in the Andromeda Key. Gemini Spark Black grew impatient collecting the Negative Energy and prematurely summoned Andromeda. However, thanks to Rockman and the AM Sages, Andromeda was defeated.
Gemini Spark Black is literally stabbed in the back by Gemini Spark White who sacrifices his other self to collect the Negative Energy and restore Andromeda to its full power. Gemini Spark White is later destroyed by Cepheus, but Tsukasa is unharmed with no memory of the ordeal.