Ninkoi - Karenbo Hichouにんこい~隠恋慕秘帖~ (Japanese); 忍恋～隠恋慕秘帖～ (Chinese)

From MangaHelpers: Mikado Shuuji is just an ordinary guy, but he has a very unusual knife that he uses for cooking. It’s actually a famous short sword that marks its owner as the head of the ninjas, and now there are hot ninja girls coming after him to take it back. Hinamori Botan is one of those hot ninja girls, but her efforts to attack Shuuji mostly end up inadvertently stripping her. After she concludes that she does not want to steal the sword and kill Shuuji, she ends up deciding to serve him as a ninja instead. Now she must protect him from the other ninja that come to claim his sword and his life!