Sumire
Sumire is the protagonist of Sputnik Sweetheart for the most part. A young woman who aspires to become a writer, Sumire many times comes off as a flighty, capricious individual. When she meets Miu, an older Korean woman, she decides to elope with her to Europe, chasing the sexual urges that she has never felt before towards anyone else.
K
K is Sumire's only close friend and is in love with her, to no avail. He is a 25 year old kindergarten teacher who represents the epitome of conformist docility, moulding himself into another person entirely to conform with society's standards. When Sumire goes to Europe with Miu, he begins to receive letters and documents from her. When he goes to Europe, he begins to piece together the story of the girl he loved.
Miu
Miu is a middle-aged Korean woman who leads Sumire around Europe. However, while Sumire is attracted to Miu sexually, Miu is unable to reciprocate in part because of an encounter from when she was younger. Miu says that she looked out the window of her apartment to see another version of herself having sex with another man, a disturbing encounter that has left her deeply traumatized and unable to have sex.
