I read about him awhile ago so I don't know if I'm super accurate but Luke is basically Sage's reincarnation in this world. Apparently he was under orders to kill her in the previous world but he had an obsession with her so he ended up suiciding or dying somehow. In this world I don't remember if he already remember her or not but he will and is still obsessed with her. He switches teams to Kasia's side. She's going to end up with Ernon so he ends up sad and single. He begs her to look at in in their third life and she agreed. He ends up suiciding again and it's implied he finally gets with her in his third life where they are modern day coworkers in the military.
Idc about how much he's obsessed with her. He's a piece of shit that should never get a chance. Disappointed in Kasia for willing to give him a shot. He's essentially one of those people that says they'll kill themselves if the person they like break up with them or w/e. Except he actually does it, while also killing them said person of affection at too. Get that toxic shit out of here.