Mitsuba is Sougo's older sister, and took care of him since their parents passed away when they were children. Sougo cares deeply for Mitsuba, and is uncharacteristically polite and docile when around her. She is acquainted with the rest of the Shinsengumi, and regularly brings them extra-spicy senbei, which she herself enjoys. She suffers from tuberculosis, and often requires medical treatment, which Sougo used to send money for. She has romantic feelings for Hijikata. Although it is revealed that Hijikata does in fact love her, he feels that he is unable to give her happiness, that is why he treats her coldly.
SpoilerShe dies in chapter 132 of the manga (Episode 87 for the anime) after a brief conversation with her brother.
She is roughly based on Okita Mitsu, sister of Okita Sōji.
Historically though, it was Okita Sōji who suffered from tuberculosis.