Autumn's Concerto- The SL is always there for the female lead and never crosses the boundaries of friendship. He kinda ends up with another girl in the end, but is in the final episode and they don't show a relationship, he saves her and to me it was more like something could begin.
Beautiful Gong Shim- She's in love with the SL at first but he's slow with his feelings, even though he's clearly interested he loses the chances. He's the sweetest though, and you root for him.
Behind Your Smile- The SL is super sweet but too slow to make a move and she falls for the ML. I think he ends up falling for another girl, but is not until the very end so is not noticeable, he just starts to see his old friend in another light.
Sassy Go Go- Which I'm watching now, the SL starts feeling love for her but the ML is his best friend so I don't think he will ever confess.
Bromance- The SL is another sweet guy whom the FL knows from childhood, he's just too slow to confess. Same actor from Behind Your Smile, this guy seems stuck in a one sided love lol
Chicago Typewriter- The second lead is a ghost that fell in love with her in her past life, in her present life he's still in love but is an impossible since, well, he's a ghost.
Doctors- He's a colleague and although he seems to try to talk to her and like make a move, he's kinda shy. I don't remember if she ever when on a date with him, but I don't think so.
Flower Boy Next Door- This SL was kinda frustrating for me because he like her for a long time but never made a move. The FL is incredibly introverted, to the point of isolation, the ML came along and helped her get out of her shell and the SL lost his chance.
Goong- I think fits the bill because he doesn't end up with a girl and falls for the lead but she's married so is impossible love.
Hi, My Sweetheart- I felt so sorry for this SL, he was there for her every step of the way and loved her for years. He was waiting for her to grow up since he was older than her but he waited to long and she fell in love with the ML.
Secret Garden- Another SL that is just to slow to show his feelings, he also deserved her but he's in a position of authority so he was kinda waiting for his perfect time.
My Sassy Girl- He's her bodyguard and she's a princess so he kinda just loves her from afar.
Strong Woman Do Bong Soon- she was in love with him first and for a long time, he realizes his feelings too late. He doesn't end up with anyone since he still in love when the drama ends.
The Master's Sun- I remember the second female lead was in love with him but I can't remember if he ever reciprocates. I know he was very in love with the FL and tried to woo her but lost to his boss the ML.
Who Are You (2013)- Very heartbreaking SLS, he's her ex-boyfriend but he's dead and she doesn't remember him, she can see him but not hear him. He watches from afar as she falls in love with the ML.