Kabamaru Igano 

Kagemaru Igano, better known as Kabamaru, was a ninja in training. After his grandfather, Saizo, passed away, he lived deep in the mountains, away from human life, barely eating a proper meal. Out of nowhere, his grandfather's former lover, Ran Ohkubo appeared! Tempted by her offer of "a proper meal," Kabamaru enrolls in Kingyoku Academy, the prestigious school where Ran serves as the principal. There, he meets the lunchbox of his dream and the principal's granddaughter, Mai Ohkubo.