As mentor and student. Since William is a noble from France and from a family of assassins per cannon, I just created a fannon that Achilles sent Connor off to learn the ways of the European assassins directly from him. It also helps her learn the ways of the colonists, grow up a little and not to depend on Achilles so much. Plus she can learn French. I honestly never thought of them as more than platonic (hmm, interesting if it was more, lol), though Connor utterly trusts Will since she did spend a year with him. Hopefully, that came through since she allows him to get within her personal space and they have an easy rapport.