Okay, I think I get what is going on.
As we learned, his summoner has the blue box system too. It isnt a coincidence she raised him, the blue box system probably compelled her.
The same applies to Rena, the reason why she probably follows the mc all the time without question is the blue box system.
And each blue box system probably has their own specialization. His summoner has the quick learning system, he has the reset system and Rena probably has the dream system.
So what happened in this timeline is, Rena had the li-lium with her, so it stayed with her. So she became an executive, but he system let her see dreams of him so she most likely either wanted to find him(and used an excuse) or assume he is trying to brainwash her through dreams.
I would also guess that necromancer has the soul absorption system. (Which she partly transplanted into him) and the succubas also had some sort of system.
Aka, the system users are attracting each other.