Nokizaru

The year is 1560 -- the Sengoku Era. The boy Asahi, who makes a living by hunting samurai deserters with his abnormally strong hearing ability, strives to one day serve as a shinobi under the "War God" Nagao Kagetora, whose words moved him as a child. A tale of life during wartime ...

(Source: TAL Scans)

Note: "Nokizaru" refers to a class of ninja that historically served under Uesugi Kenshin, whose original name was Nagao Kagetora.