I would imagine that it produces strain relative to the extremity of use? Maintaining a magic spy network shouldn't be effortless, rather than reducing its power it should probably be counterbalanced with a large downside.

B Ranked Strength isn't uncommon with Assassins, considering True Assassin had it, but also take note that Stay Night/Assassin had a crazy demon arm, while yours just has fancy eyes. I would suggest reducing strength due to your character being more of the subterfuge type, rather than the direct combatant that SN/Assassin was.