As far as I know Masaru hasn't actually been investigating why he was the target. Even if he has, it doesn't involve any characters that aren't or don't plan to be doing anything except I guess whoever hired them (Was it Agent 00?).
I'd rather not have Kazuma involved because he's already going to be doing so much stuff, it doesn't get anyone who isn't doing anything doing anything.
Honestly, Armin's whole purpose is to just randomly create intrigue-related plots when we need to get some characters doing something. I mean, he doesn't do anything otherwise and I have no plans for him to "develop as a character." Unless someone wants to befriend him or something.