MariRei's Story
What led to their love?
Between the events of S1 and S2, Mari starts their own esper-witchcraft business, right next door to Reigen's office!
A red light pops in Reigen's mind, recognizing the danger to his business. So, what does he do? He starts an investigation, carefully analyzing their methods, strong points and weaknesses, tryigntrying to crack their business open and get rid of them once and for all. When he's busy, he asks his student, Shigeo and his friend, Dimple to question Mari. Mari decides to respond to Shigeo's questions, but they pretend they can't see spirits like Dimple.
What neither of them predicted is that Mari would grow fond of the kid. He often visited for a cup of tea, or some advice, and they were pleased to help him out. One day, Mari reveals to Shigeo that they are an esper themself. He suggests they join him and his master during their upcoming business trip to help out exorcise an evil spirit. They were a little skeptical, considering they'd have to deal with Reigen, but they agreed.
Season 2 OVA spoilers!
Well, let's just say it didn't go as planned. During their trip, Mari and Reigen got stuck together in the evil spirits realm. For them, it felt like days, and during that time, they managed to exchange a few words. Before this, most of the exchanges they had were small talk and side-eyes before work. It felt odd, for Mari to find out that Shigeo was honest when talking about Reigen. They learn a little about each other, and after that little experience, they dare to say more than one word to each other at work.

They start talking. Hellos before they enter their offices, going for smoke breaks at the same time, visiting each others offices for a quick lunch break or tea. And that's all, for a few months, at least. They hoped nothing would change.
And yet...
A crush develops. Their witty exchanges related to their rivalry have them quiet in their offices later. A laugh makes a heart skip a beat. Sometime, Reigen looks at their MobBook profile and makes triple sure he doesn't like any posts. Sleepless nights start, and it gets weird. Because none of them wanted it to be like this. We're supposed to be enemies! The tea breaks were already too much!
The tension gets extreme, they barely talk to each other just to avoid distractions at work. And yet, these walls are thin, and they can hear each others voices. Reigen decides to do something, and enters Mari's office. He knows they have clairvoyance skills, so he asks them to look into the future. To not be too obvious, he asks about career first, and despite the hesitation, he asked about love. They foresaw a great future for his love life, and they both knew what it meant. Reigen leaves.
Ok, what now?? Neither knows what to do, and honestly everyone is annoyed by their behavior. Well I haven't really thought of how this would go, but I imagine a drunk first kiss, an interruption and another week of tension before they get together lol.