I'm actually kind of sad this is going to be a harem, because the wholesome tomboy childhood friend/girl next door facet to this is really, really likable, and I actually want an entire story that's just about her being the hero and him trying to catch up to her to keep their promise. This is actually good enough that I feel like the traditional sort of harem shenanigans would only get in the way of a really great vanilla love story.