Otsu is one of the main protagonist in Vagabond and the main love interest to Musashi.
Otsu is kind and caring but she secretly desires intimiacy and a loving family.
Otsu loves Musashi ever since they were children but she was unaware of her feelings due to her arrange marriage to Matahachi.
She thought she was in love with Matahachi because he was the only person to continue his friendship with Musashi.
As a baby, she was abandoned in front of the temple along with a flute. The monks took Otsu in while Hon'iden family raised her with intentions of her marrying Matahachi. She is a also childhood friend of Takezo and Matahachi.
The marriage between Matahachi and Otsu go awry when Matahachi runs off after the Battle of Sekigahara with Oko to Kyoto. Angry at Matahachi, Otsu refuses Granny Hon'iden's offer to join the Hon'iden family and seeks life and love elsewhere.
Originally she planned to leave Miyamoto with Musashi but Musashi decline saying she'll disrupt his training and she went to the Yagyu clan.
Currently she is travelling with Takezo's "apprentice" Joutarou in order to find him.