So I have a 100lvl Reshiramin my black version and I want to trade it to sun right as is start the game. I mean I just made it to the second island and I want to transfer it with pokebank. Can I do that and will I be able to use it?
You should be able to transfer your Reshiram over to Pokemon Sun. Pokebank will let you, and as for the game itself, it should let you keep your Reshiram.