A well-crafted montage would work, you could dedicate an episode to a few mini sequences of various things they got up to in their youth, which bought them closer together, leading up to the first kiss or something.
That said though, even if it's a fantasy/action, you don't need to shy away from romance. At all! For me, the depth of character interactions like that are what take an interesting story from being merely interesting, to being properly compelling.
I'm not going to shy off romance in my series, and it's sci-fi/fantasy/action. I'm just going to sprinkle it in all over the place, working it around the plot so that the people there for the action don't get bored.