Playing the Villain Boss in Three Comics at Once

You transmigrate into a new world brought together by three manga series:

Adventure manga:

The protagonist is a rookie investigator tasked with dealing with all kinds of paranormal traces. You're the mysterious senior he happens to meet in a library, and you occasionally show him the ropes.

Slice-of-life manga:

The protagonist works for a government agency, handling the aftermath of paranormal incidents in the country. You're a close friend he invited to his country, and your current main job was also introduced to you by him.

Fantasy manga:

The protagonist is an undercover agent embedded in a terrorist organization, tasked with passing along intelligence and protecting his country. You're a temporary member of that organization, responsible for cleanup and tying up loose ends, so you often run into the protagonist at crime scenes.

*

Now a background character in all three manga, you are bound to a System that showed up late. It wants you to become the final boss in all three series.

You smile and turn it down, because all you want is a quiet, ordinary life.

However...

The System offers a reward you can't ignore.

Shamelessly, you accept. You plan to have a spectacular showdown with the three protagonists, then make a clean getaway. But first, you plan to work overtime, stop by the supermarket, and take a side job before you start preparing.

In short, you're putting it off indefinitely.

The System stares blankly. "...Do you have chronic procrastination?"

*

But before long...

After investigating to the very end, the adventure manga protagonist discovers that his teacher created the paranormal phenomena?

After handling things to the very end, the slice-of-life manga protagonist discovers that his close friend is a spy?

After going undercover to the very end, the fantasy manga protagonist discovers that the boss is his subordinate?

The three protagonists: "Impossible! He must have his reasons!"

Theme: Never betray the friendships of youth