Keizen

Keizen (Castor, Shining (炯然, Keizen) is a remnant of a famous shinobi, Sōji (双児, Twins, Gemini). Originally a Shadow Clone, he didn't disperse when the original user was slayed, instead being imbued with the energies of life and form alongside Daikan. For unknown motives however, the slayer of Sōji decided to brainwash the two, giving them new names and allowing them to develop new identities.

Dubbed Keizen by his-would-be killer, this shadow clone would find himself in the Land of the Sky, ignorant to what had happened to him. Beginning a new life, he would fall in love with the technology of the advanced shinobi there. Becoming a scientist, he would devote his life to the making of technology and studying the sciences, as well as being a shinobi.

Adjusting to his new life, he would develop a mastery over his bio-electrical systems. Along with that, he would seek to fuse with the technology he so loved, turning himself into a mecha of sorts, almost like the Asura Path to a degree. Developing into a powerful shinobi with a new life, he had no clue about his past.

Eager to find out more, he would eventually find himself face-to-face with Daikan. Initially brawling against each other due to different views, Keizen and Daikan both survived the day. Both now eager to decode their mysterious origins, find themselves pitted against each other in a quest to figure out their paths, which both suspect are connected.

Keizen wants nothing to do with Daikan, but both must realize that in order to finally solve their puzzle of a past, they must ally with each other as brothers to put an end to the cruel oppressor that has led them through this strife and conflict.

Trivia

 * Keizen's namesake Castor (Shining) is a reference to the Dioskouri Twins, Castor and Pollux. Named in Latin as Gemini, his original's name is the Japanese translation of Gemini (Twins). Daikan is named after Pollux. See Castor and Pollux on wikipedia.