The person who approached sasuke had no shoes on and Karin had no shoes on either, it might be herAre you talking about who is inside of Spiral Zetsu? I believe is Yamato. I don't see any other logical choice.
And about the mysterious person approaching Sasuke. I think it may be Kabuto.
If what Itachi said about Izanami, then Kabuto could have possibly found his way, and realized that he should help the alliance, and not fight it, he could of just arrived to see Sasuke unconscious. Who knows though, who else could it be????