Claus

There is one sure way to get her to marry Cube, but it is a bit extreme.

If your daughter loses from one of the "bandits" (there is one in the mountains,
lake and plains), there is a chance he'll try to rape her. In that case, Cube comes
and saves your daughter. Each time he does this, the cube/daughter relationship
raises tremendously. Do it a few times for a guaranteed cube/daughter marriage.

The positions of the bandits are:
Plains - inside the ring of stones, near the end of the area (keep jumping in and out
of the whole that leads to the slim milk)

Lake - walk around the big flower

Mountains - Try going to the house where they say "there is no one here"

To get the lake bandit, you must defeat the plains bandit, and to get the mountain bandit,
you must defeat the lake bandit. I think.