>>713167197
To fulfill whatever promise he made, even if it means having (You) in control of him. The SOUL - our soul - is needed for SOMETHING. We just don't know what it is yet. It could be said that it's needed for sealing the fountains, which is true. It's our power that does that. But what would be the point as the Knight? To just go around opening fountains and having us close them forever? It doesn't make any sense.
The answer I'm leaning towards is that Kris doesn't necessarily need our soul, but that SOMETHING ELSE does. Kris is just the only one, as a human, that can carry us around until the situation arises.