Otsu (おつう, Otsū) is one of the main characters in the Vagabond manga series, and Musashi's main love interest. She is an orphan who grew up in Miyamoto village, and is childhood friends with Musashi (then Takezo) and Matahachi.
Otsu is a classical Yamato Nadeshiko, or 'Japanese beauty' with a slender build, fair skin and large, expressive eyes. She has long hair usually tied up at the back, wears a kimono and sometimes a basket-hat.
Otsu is kind, caring and independent, but she secretly desires intimacy and a loving family. She grows very attached to the people she cares for, such as Sekishusai Yagyu, whom she views as a grandfather, and Jotaro, who becomes like a little brother to her.
- Musashi Miyamoto: Otsu and Musashi were childhood friends, and she has loved him ever since, but she was unaware of her feelings due to her betrothal to Matahachi. Despite Musashi's commitment to the Way of the Sword, she continues to have feelings for him.
- Jotaro: Otsu comforts Musashi's would-be apprentice after Musashi leaves him behind at the Yagyu estate. They begin travelling together and form a sort of big sister/little brother relationship. She refers to him as 'Jotaro-chin' or 'Taro-chin.'
- Soho Takuan: Growing up at the Shippoji Temple in Miyamoto, Takuan was something of a big brother figure to Otsu. He continues to be a friend and mentor to her, as he is to Musashi, throughout their travels.
- Matahachi Honiden: Musashi and Otsu's childhood friend, Otsu was betrothed to Matahachi until he runs off with Oko to Kyoto after the Battle of Sekigahara. Feeling betrayed, Otsu declares she hates him and refuses to have any more to do with him.
- Osugi Honiden: As Matahachi's betrothed, Osugi helped care for Otsu when the latter was growing up. However, after Matahachi abandons Otsu for Oko, Otsu refuses to join the Honiden family, drawing the ire of Osugi. Thus ire turns to hatred when Osugi learns Otsu has run off with Musashi. While Otsu remains initially grateful to Osugi for helping raise her, she becomes angry when faced with the old woman's hatred.
- Sekishusai Yagyu: During her travels, Otsu finds employment with the Yagyu family as a servant to their elderly patriarch, Sekishusai. The old swordsman informally adopts her as his granddaughter and gifts her a dagger with the Yagyu clan's emblem. Otsu declares she will always think of Sekishusai as her grandfather.
As a baby, she was abandoned in front of the Shippoji temple along with a flute. The monks took Otsu in while the Honiden family raised her with intentions of her marrying Matahachi.
When Takezo returns to Miyamoto and begins killing the men hunting him, Takuan devises a plan to capture him with Otsu's help. Witnessing Takuan's cruel treatment of the captured and exhausted Takezo, Otsu takes pity on him and eventually helps him escape. She plans to leave Miyamoto with Takezo - now renamed Musashi - but Musashi declines saying she'll distract from his training.
Following Musashi's rejection, Otsu finds her way to the Yagyu family and is brought into the service of their ageing patriarch, Sekishusai.
Otsu meets Jotaro, who immediately reminds her of Musashi as a boy. She soon learns that Musashi is Jotaro's master. When Jotaro kills the Yagyu's guard-dog in revenge for attacking him, Musashi faces down several Yagyu disciples. It is Otsu's flute-playing that cools his bloodlust, and for a moment the two reunite, with Otsu confronting him over his desire to face Sekishusai. Later Otsu finds and comforts a lost Jotaro, who has been left behind by his master. Together they leave the Yagyu in pursuit of Musashi.
Otsu and Jotaro overhear a gang of samurai planning to go after Musashi. When the men try to apprehend her and Jotaro, she is able to defend herself, styling herself as Sekishusai's granddaughter. She and Jotaro later encounter a sick Osugi and help care for her, although Otsu becomes angry when Osugi restates her hatred for her and Musashi. She attacks Osugi, but is stopped by Jotaro.
Otsu and Jotaro are in Kyoto, and among the spectators for Musashi's duel with Denshichiro Yoshioka. When Musashi is victorious Jotaro is eager to reunite, but Otsu refuses.
Otsu plays the flute quite well, and would do so for Sekishusai during her time with him. Her flute playing managed to draw Musashi to her after his fight with the Yagyu disciples.
She has some knowledge of self-defence, learned during her time with the Yagyu. She is able to defend herself against a group of ronin trying to intimidate her and Jotaro.