On the Topic of Hansu and Isak...

Hansu and Isak were the two romantic interests for Sunja up to this point in the story, and are vastly different characters. 

The first romantic partner that Sunja had was Hansu, a wealthy and relatively powerful Korean who had been living in Japan. His interest in Sunja initially read as somewhat strange, being that he bothered her on shopping trips like clockwork, and Sunja seemed to think as much too. Her attitude shifted quickly after Hansu saved her from sexual harassment by Japanese high schoolers, and they entered into a relationship. Afterwards, it read like many other young love stories, as Sunja became pregnant and with Hansu unwilling to become married; of course, the details were different from the atypical young love story, as Hansu was a great deal older than Sunja, and was already married and with children in Japan. Although Hansu was enthusiastic about helping Sunja, willing to buy her a house and still continue their relationship barring marriage, Sunja felt her world come crashing down, as a single mother was culturally unacceptable in Korea at the time.

The individual who got her out of the predicament of being a single mother was Isak, a good-natured minister whose life was saved by Sunja and her mother. In stark contrast with Hansu, who was well-built, Isak was chronically ill, and soft-spoken compared to Hansu's straightforward approaches. That being said, some of their circumstances were similar: Hansu was wealthy where Isak came from a high-class family, though they were facing financial troubles at the time of the story. Both characters were kind though somewhat uncompromising, with Hansu being certain that the arrangement he offered Sunja was the best for her, and with Isak creating a relationship that wasn't necessarily romantic from the outset.

Hansu was an odd character to pin down. His actions were definitely intolerable by modern standards, which made them even worse for Sunja in the early 20th century. What's weird is that he reads like a character that didn't come from a culture where being a mistress was a shame upon one's lineage, and his offer seems like a genuine attempt to further his love for Sunja. He's still scummy, but it feels like he was written with an unnatural amount of nuance for his actions.

Isak is more clear-cut. He's the stereotypical "good guy" without ulterior motives, saving Sunja from dishonor in part due to the fact that she and her mother saved his life, and in part due to his innate kindness. Granted, being a "nice guy" isn't always what someone is looking for in a relationship, and that seems to be the case with Sunja. I think it would be best for Sunja to stay with Isak, but the way things have been written thus far seems to imply that will not hold true.