The mysterious person is someone that we haven't seen before.
Ok I'll get straight to the point:
In this page we see that Hashirama clearly saw somebody. If you go by saying that this man is Madara himself, I'll say no because he can't have just used Izanagi. The reason for that is that we have already seen how it works (Danzo's case) and at that time whenever Danzo was supposedly about to die his "false" him had been disappearing. Therefore, if we were to say that Madara used Izanagi, it's obvious that he didn't because it didn't apply the same way it did on Danzo. Furthermore, Hashirama's certainty based on his vast knowledge of his rival's capacities leads me to believe that he says the truth.
Being said all that, that person must have been a real individual. Ofc, he can't have been a zetsu clone since Madara hadn't yet obtained Hashirama's dna and so he couldn't have started making experiments by creating Zetsu clones. If we agree that that "something" was an actual individual, then we can also agree that no one could have survived after such a long time to be the one standing behind Sasuke right now, well that's it unless he has some extraordinary vitality. Before, I go any futher I would like you to examine very carefully that guy's shadow:
I don't know about you guys but for me his shadow seems to bear some very distinctive characteristics. On his shoulders it's as if you can see very small flames while his head looks as if he has 2 horns one small and one bigger. Now you might say that this is completely subjective but we can all agree on one thing, this shadow is not completely normal, it has that something, call it whatever you want. Furthermore, as for those who'll say that this person is Kabuto, I'll answer no for 2 reasons. First Karin doesn't recognise his chakra even though she has come in contact with him, secondly, even if you say that his chakra has been altered because of his experiments, you can't just pass by Karin's description: has a really terrible feeling to it! Even if he was Kabuto, his escape from Izanami would mean that change of his mindset and therefore his "evil" chakra. If you go by saying that he's jsut another Spiral Zetsu I would answer no for reasons of aesthetics. Right now, Spiral Zetsu is a huge mystery and you just can't throw a bunch of them, it would ruin the whole atmosphere. Besides, the whole point in him is that he's unique, if more than one had managed to be created, why wouldn't Madara or Obito use them before?
Based on all that, I think that the person Hashirama back then and the one Karin saw now is one and the same. More importantly, I'll say that he's from the uzumaki clan and that's the reason he has survived this long. Even the the amount of time is too big, I do believe that Uzumaki can find ways to prolong their lives this much. Ofc, this is just a hypothesis. In any case, the reason I'm saying this is because of Hashirama's certainty back then. Having confirmed that that thing he saw back then was an actual individual, and being certain that he can't have lived for so long without being a special case, I believe that it's impossible for him to be a normal person from Madara's and Hashirama's era.
I know that my theory is quite shaky but this time I'll believe my instinct. Being someone we already know and whom we all have predicted is just too cliche and something that Kishi doesn't want to. A prime example of such certainty was our belief as to how Minato would have given Naruto the other half to become complete or the other half to just surive. In both cases we were utterly mistaken. Unless, you go by questioning Hashirama's certainty after all the battles he had against Madara, you just have to accept that at that time Hashirama did see something, it wasn't just some wind that blew.
Conclusion: The person Hashirama saw back then and the one Karin felt now is the one and the same.