Why does Otama decide to become a mistress? Take her social status, background, and Japanese culture into consideration.
Otama has no choice but to become a mistress to Suezo because of her social status. She is not from a wealthy family and she is being raised by her father after her mother passed on when giving birth to her. Suezo is a wealthy man and according to Otama he is capable of taking care of her father. Suezo is generous towards Otama's father and this helps him keep her as a mistress.
Otama's background coupled with Japanese culture compounds her problem. This is after she gets married to a policeman who does not disclose that he is already married. This information gets to Otama and this leads to the dissolution of her marriage. Although she was not at fault the situation was going to make it difficult for her to get decent marriage proposals according to Japanese culture. This also made it easy for her to accept Suezo's proposal given that it is a rare opportunity in a bad situation.