Difficult to judge if the romance is a sub-plot of the series or not. Usually there are other plots besides romance, but which plots play a larger role I can't really say, it's very subjective.
Because you say it should be "happy ending", listing some series might be a spoiler as that gives away how the series ends.
So I will list it in a spoiler format:
- Flower of evil (big thriller)
- Eulacacha Waikiki (somewhat fits that bill, but is mostly comedy)
- The King: Eternal Monarch (hey I liked that series)
- Bring it on, Ghost (fun story I felt like)
- Pinocchio (fits your bill perfectly I feel like)
