Board Thread:General Discussion/@comment-32930821-20170823152312/@comment-33862372-20171204131111

It was implied by Chara at the end of the Genocide Route. I posted this answer elsewhere, too:

She says, "Thank you. Your power awakened me from death. My “human soul”. My “determination”. They were not mine, but yours. At first, I was so confused. Our plan had failed, hadn’t it? Why was I brought back to life? . . . You. With your guidance. I realised the purpose of my reincarnation. Power. Together, we eradicated the enemy and became strong."

 There's another reason, though, and that's that, if you decide not to erase the world at the end of the Genocide Route, Chara says, "Since when were you the one in control?" which suggests to me that Chara and the playable character are somehow one and the same while also distinct somehow. Chara is somehow in control the whole time which is another tic-mark in favour of the reincarnation view.

So Chara and the playable character, Frisk, share a soul and determination, as stated earlier. Sharing a soul? That's textbook reincarnation, and I don't think that that's reaching too far. Also, I mean, Chara specifically mentions reincarnation, and that can't mean simply coming back to life in the same body. Or, I suppose it could, but when she says that she and Frisk share a soul, that puts a pin in it for me, personally.

Let me try giving another example while outing myself as a huge nerd: In InuYasha, Kagome and Kikkyo share the same soul. Kagome is the reincarnation of Kikkyo. Nevertheless, they have two separate identities and are two different people, totally independent of each other. So the way reincarnation works, in some traditions, is this way: Same soul, different person. That's what it boils down to, and I think the situation with Chara and Frisk fits the bill, especially when, as it seems to me, Chara heavily implies it in her speech at the end of the Genocide Route.