It was revealed that Gai was the original Adam, the mate created for Mana. The plan was to resurrect Gai in order to get him to obtain the power of kings from Shu which meant severing his arm. Haruka Ouma, Shu's step mom went along with this plan because she didn't want Shu's fate to be the fate of the King.
Gai also mentioned to Shu in the final battle that they would not allow Mana to just die in peace, they would keeping resurrecting her. Gai was in love with Mana and believed that to free her he must of offered to be her Adam and die with her.
Gai revealed to Shu that he was the true "King" and that Shu was nothing more than an Upsurper and pretty much this whole time was a victim in this whole mess that kept just getting chosen.