Nishiyama Chizuru was the caretaker of Odagiri.
She took care of Odagiri during his childhood. Odagiri was left to the care of his paternal grandparents by his parents but since his grandparents were barely capable of taking care of themselves, Odagiri had to depend on a young girl who lived in a nearby farm; Nizhiyama. Nishiyama basically raised Odagiri throughout his childhood.
She eventually eloped with a man who later abandoned her. She died of tuberculosis while trying to get back to her hometown.
Odagiri states that Nishiyama was "the one person in the world who accepted me" and had a strong attachment to her as can be seen in Episode 12 where ,during a mission,he gets distracted by Nogami Yuriko who he believes to be a look-alike of NIshiyama.