One idea could be a story where Mash teams up with an unlikely ally from a rival school. They start off not getting along but through various challenges, they learn to work together and combine their unique skills.
Another option could be a student from a different magic school who is jealous of Mash's popularity and unique abilities. This student could use dark magic to try and bring Mash down.
Naruto could see death magic as just another form of power that can be used for good or evil. If he witnesses it being misused, he would be angry and try to stop those using it in a harmful way. He might also try to befriend those who use it for good and learn from them. He would adapt his fighting style to incorporate defenses against death magic, like using his shadow clones to test its limits without putting himself in too much danger at first.
It really depends on the particular manga. Some depict Mash having the ability to use magic, while others might not. It all comes down to the creator's imagination and the story's requirements.
She might be initially stunned. After all, Percy is a hero and his well - defined muscles could be a sign of his strength and capabilities. It could make her blush a bit, secretly admiring his physical form while also feeling a bit flustered as she tries to maintain her usual composed self.