It was one of my favorite dramas of 2018, and I'm quite looking forward to the next season even though the main actor isn't returning. I actually had a similar reservation about watching it initially - it wasn't because I thought she looks thirteen (because to me, the actress looks a bit younger than her age, but not that much younger), but something about her in the trailers had me convinced I wasn't going to care for the character or relationship much. I was proved very wrong. She ended up being my favorite character, and the relationship is one of my favorite aspects of the drama (despite the fact that, at least so far, they're not very demonstrative in terms of physical affection).
There is a love triangle, but it's resolved nicely enough. If you're interested in romance as a primary element of the drama, this isn't really the drama for you. Although the main relationship is important, the focus isn't on romance. It's much more of an action-oriented drama with some elements of fantasy and some political intrigue.
I don't know your taste so it's hard know whether or not to recommend it, but I think it might worth a try since it's a well made drama overall.
