In the 8th and final year of the game after the second semester, Mouri may propose to your daughter while they are on a date. In order for this to happen, Mouri's orb level must be at its absolute highest. Also, you must not have triggered any other endings. If you accept the proposal, the two will marry at the end of the game.
In order to meet Mouri, your daughter must have accepted Mouri's scouting event. You can get scouted by Mouri by going to town with charm over 300 after entering junior high. Mouri's orb will be red from the beginning and there is no friendship event associated with Mouri.
There are two endings that one can achieve when marrying Mouri, a good one and a bad one. One achieves the good ending by having low morals (under 80), and the bad ending by having high morals (over 80).
In this ending, your daughter will support Mouri working in the talent agency and be a wonderful housewife.
In this ending, your daughter will be fed up by Mouri cheating on her and return home.