The thing about not picking kamui right now is that there won't be a dark tank for a LONG time. 21 is the only other dark tank in the game.
Yes, Lee's pretty good, but with Alpha (plus all the other A phys dps units) you have alternatives. As for Karen, she's also the only fire dps, but as other anons said, if you don't also pick S nanami to go with her, you're missing a fire tank to go along.
It's basically choose between futureproof with kamui or be strong now with karen.>>527568
I'm not sure if they stack, but both aife and darwin are very good general choices. Finish the memory rescue stages for the second darwin or run something like 4 aife 2 erwin for attack boost/4 aife 2 mozart for matrix damage boost. Remember to focus on upgrading 4/5/6 for atk/def