I have no problems with Jo Hanna. What I see are two teenagers viewing what a relationship would be through teenage eyes. That's forgivable. So far, the four years argument has not impressed me. Even if Irae and Kang were exactly the same age they would face these same issues (life goals, not knowing where they will live, independence, financial instability, do they want a family... what kind). These are things most people go through when they are young. To me, what makes a good story is seeing how they deal with these things and also how they try to make a relationship work.
Irae has been shown to be uninterested in thinking about a relationship with someone else (or more exactly unaware of it until it's brought to her attention). That's fine as there is no requirement for that in her life. Kang has been thinking about this so maybe it's his time to try. That point arrives at different times in different people's lives. If Hanna is able to persuade him to see her as a romantic partner then I see no reason to dislike her for it. In honesty, I think seeing them try and seeing them mature as a result would be more interesting.