The poem, set at the time of the first atomic bomb dropped on Hiroshima in 1945, has two distinct main characters. First is the narrator of the poem, a doctor. The poem is written as his journal entry. A second main character is his wife, though she plays a much smaller role. Though not one character, a case could be made that a third main character, even more important to the story than the doctor's wife, is the group of citizens described wandering in shock after the explosion of the bomb.
The doctor is calm but bewildered by the shock of the instant destruction around him. His state of nakedness after the bomb strips them all of their clothes seems to symbolize his feeling that they are all vulnerable. He might also be described as caring, as his concern is for his wife first and then his staff even though he was injured.
The doctor's wife is very involved in his description of the moments after the bomb hits but is not very well described. Still, in his description we realize that she is frightened, injured and probably in shock. She complies with his wish that she walk on to the hospital ahead of him so she may be described as traditionally obedient.
Lastly, the citizens as a whole make up a last main character. They are described as wandering naked, unaware of their surroundings or naked state and so injured that they cannot bear to have their skin touched. As a group they are important main characters that allow the reader to experience the enormity of the destruction and pain caused by the bomb.