Both are known to be katanas that are hard to handle which only a few people could master - including Okita Souji.
Okita Souji took Kashuu into battle with him in the Ikedaya Incident (1864), wherein the Shinsengumi went into armed conflict with the Jouishishi, who opposed the Tokugawa Shogunate. It is said that Kashuu’s tip broke during the battle and he was deemed unrepairable, so he was abandoned by Okita.
Since Kashuu Kiyomitsu was born in a poor environment, he cares a lot about his appearance. He thinks having a clean-looking appearance will earn him his master's love. The mole by his mouth shows his indescribable, bewitching beauty. (from official twitter)