Mrs. Yokokawa

She is first seen tying a hood on Setsuko, telling her to be a good girl and to listen to her brother as the family prepares for the bombing. She then states that she best start for the shelter, and for them to meet her there as soon as possible. Seita asks her if she has her heart medicine with her, and she says yes and motions to her bag, telling him not to worry. She then leaves.

When Seita and Setsuko arrive at the shelter, Seita is told his mother had been badly injured. He rushes to see her and finds her asleep on a bed, heavily bandaged, bloody, and burned. Horrified, Seita painfully states that his mother has heart trouble and asks if she can have her medicine. The man there states he will find out. When he leaves, Seita tries to communicate with her, but is greeted with only soft breathing.
When Seita returns to Setsuko, he tells her that she is hurt and in the hospital, but will get better soon, afraid to tell his little sister the truth.

Seita's mother's dead body is then seen being put into a truck as flies and maggots move about her, while a man states that she is not unbandaged because her body is too gruesome to see; another man states that she must go on the truck that day with the heat they are having. Seita watches them carry her away. She is then cremated in a huge fire with many other bodies.

(Source: Hotaru no Haka Wiki)