Don’t be stupid, if the MC had no connections to anyone outside of his grandpa, his brother and revenge, he’d be untouchable and there would be no actual conflict or suspense. However stupid the FMC may be, she provides openings in his cold defense for him to be or become vulnerable enough for there to be suspense. You WANT the enemies to catch on and retaliate, To show you what they’re capable of. He didn’t learn all of this cool shit just so he could pick off some jerky unknowning highschoolers. This should be a battle of Cunning minds utilizing flaws in character