Id like to know the name of this anime ill try to explain it the best i can.?

Well ill try to best explain it , its not a very long anime maybe like 10 or so episodes but it starts off with two twin boys landing on a prison type planet and one is pretty much immediatley killed off. the other one sorta survives through everything his weapon is a little dagger type knife and on the planet only the females are aloud to choose which guy they want. and the way to become the leader of the planet is through fighting and the main character ends up winning a fight against the current leader with the help of this other sniper dude who distracts him with a stray shot so he can go in for the killing blow. time goes on and what not and the main character finds out the little kid he first landed on the planet with wasnt actually his brother its just some boy he was with and he apparently isnt human but he has the ability to adapt to best suit the enviroment he is in and since he was with that little boy he pretty much just changed into him to suit the current moment. but yeah i dont know how much else to explain it chances are ill probably never find or hear the title of this anime again.