Yukihiko is one of the former Legendary Pokémon who returned to his human form due to the machinations of this AU’s main evil team/antagonists, Team Oracle. The exact details of how he and Lorelei met and ended up married is still something I’m working on, but either way they get together and have one child, a son named Finn. 

I think I’ve mentioned this previously, but Team Oracle is targeting both the former Legendaries themselves and people connected to them, including their children. While on their pokémon journey, Finn and his childhood friend Rihito (the son of former Entei and Moltres) are take prisoner by the team to be used in their plans, though when this happens nobody knows that Team Oracle is responsible as they’d stayed mostly underground up to that point (it’s roughly around the same time Emery and co. start their own journey.)

Losing their only child under mysterious circumstances with no leads on if he’s even alive puts a big strain on Lorelei and Yukihiko’s relationship, which eventually leads to them living separately. I might not have them actually finalize a divorce, but for all intents and purposes, they’re no longer together. However, when Finn (and everyone else who had been captured by Team Oracle) are rescued during the main plot, there’s a good chance his parents will get back together.