Kagami - Isshun Hajimari

Kagami Arata Amateru is a cheerful, energetic 14-year-old boy who lives a simple life traveling and trading with his grandpa Genzo. Everything changes when they are attacked by the infamous vile shinobi Kurogane, a former Sōke turned rogue.
A mysterious, powerful shinobi named Jinsei (the "Unavoidable Step") saves them and reveals that Kagami has been hunted since birth because of a dangerous hidden power inside him. Jinsei brings Kagami and Genzo to Takayama, a powerful shinobi alliance and institute that serves as a beacon of peace and strength in Tsukikure.
At Takayama, Kagami begins his life as a Shoshin (new student). He meets Rein Hoshiko Amateru (a determined girl from the same distant clan), the laid-back but talented pervert prodigy Takumi, and the strict but caring Giin Aiyame.
As Kagami trains hard (learning Shunpō, Mizugumo, Shadow Projection, and refining his Hikari Dama Kyōka), he discovers the weight of his secret: he is connected to the ancient god Izanagi. This makes him a target for the shadowy organization led by Kuromaku.
Meanwhile, a mysterious white-haired boy named Shirō (an Itsunagi with powerful Shiryōken) enters Kagami's life as a rival and eventual complicated teammate. Other key figures include the quiet wolf-like Tsubaki, the gentle medical shinobi Iroha, and the complex leadership of Takayama (Gichō Shūen Mizuhara, Masatora, etc.).
The story blends action, training arcs, emotional drama, light romance (Kagami & Rein), and rising political/shinobi threats. It has strong rivalries, tragic backstories, and a rich world with clans (Amateru, Itsunagi, Raiketsu), kami's, and forbidden powers.