While these shows represent two different genres (To Be a Better Man is about Friendship, and Reborn is a crime drama), both depict severely depressed men who strike up a friendship with orphaned girls.
Both are about grown men around their 40s who suddenly find themselves having to take care of a teenager. They are forced to grow up and mature in order to do so as well as navigate through the hurdles of their own lives. Old Boy is more of a slice of life. Both dramas have a similar vibe.