In Episode 13, about 49 minutes in, when the writer Jae Yeol has just started to wonder about the existence of Kang Woo, he's talking to him on the phone, and walks up to some window shutters with a design of triangular cut-outs and opens them a bit, but then just looks through and they show him from the outside, framed by one of the openings. He's looking for the girlfriend/doctor Hye Soo, who has gone downstairs to talk to his friend.
Just a few minutes earlier, she had seen the jigsaw puzzle of them that their friend from the house had given to him, for explaining that mental illness just messed up a small percentage of a person's mind, for a while, not all of their life. Fragments.
So he's showing through this triangle, fragmented. The image of Kang Woo, riding no-hands, eyes closed, on a bicycle, is put side-by-side. The shirt he's wearing shows him as the center of a starburst of fragmentation-looking lightning bolts. It seems to show not just fragmentation—though in color and shape, they do kind of match the triangular window—but maybe life and power and pulsation. In that scene/imagining, he seems strong and confident.
Third viewing, February 2017, I noticed more triangles, from late episode 13 (I think) on.
After Hae-Soo finds out about the problem, after he has asked her to marry him and said she loves him, her earrings and finger rings start to change with her mood and confidence in various scenes, it seems. I might think it was nothing, just coincidence, except that they show her ears, or her fingers, pretty clearly. And in one scene there's a glint of light off a triangular earring. So I could be wrong, but it would be quite enough to write a paper on in a film class. :-)
I think they're to show her fluctuation between confusion and hopefulness, fragmentation (the triangle was already used) and strength (a circle of pearls is... whatever the circle represents, fully reinforced).
The first triangle I noticed. Above that is a cirle with one pearl, seems to be.
I don't think those are coincidental. If they are, I won't be the first human to read meaning into chance happenings. Of all the earrings in all the Korean dramas, though, these are simple shapes, simple designs, and changing with the emotions of someone who's already in a triangles-mean-something scenario. :-)