Being Kiriya your fiance, you helped the Kakushi's take care of others' wounds, although everything had a slightly tense atmosphere. It was not unusual to get married at a young age, the Ubuyashiki clan always did that.

After the battle against Muzan, Kiriya was left in charge of the Demon Slayer Corps: He was raised for it in advance. With Muzan and the upper moons dead, Demon Slayer still exists, as there are still demons loose around there.