I don’t think you can term a romance between 2 humans and 1 ghost “normal” in any sense. Especially when Seol (until only very recently in ep12) can’t even see Yul (or Jin-oh). Prior to his sudden materialisation, Yul is very much a non-entity in Seol’s relationship with Se-joo.
My sympathies lie with the invisible entity – Yul. We know he loves her, probably juz as much as Se-joo (or Hwi-young in his previous incarnation), but in the present, he can only play bystander to their budding romance. So I wince when his excitement at having found out the secret to making himself appear in front of Seol is swiftly crushed by Mdm Wang’s advice. She lays it out quite bluntly that as a ghost, Yul has no business in the human world. Not only is he incompatible with Seol, his desire for her may empty him out and turn him vicious. I can only imagine Yul’s disappointment being compounded when previously in his human form, he also never managed to woo Seol successfully.
But the unfairness to second ship Yul doesn’t make their love triangle any less funny. It is still a case of 2 guys competing for a lady’s attention, only in this case, we see 2, but the girl sees 1. Much hilarity ensue when they go for threesome dates. From Seol’s perspective though, it is only her and Se-joo. I don’t blame her utter confusion when Se-joo starts acting funny, like snapping at her, when we know it’s Yul he is annoyed with. Likewise, when Seol praises Yul as the “handsome guy” whom she remembers from her past life as her shooting instructor, she doesn’t understand why Se-joo has this annoyed look on his face – it’s cos he (and we) sees Yul mooning at Seol.
While Seol may be the target of affection from both men, sometimes I feel Yul and Seol are sharing Se-joo. After all, he is only one who sees both and can converse with both. In a way, I think Se-joo is pretty happy with this arrangement. Sort of like he gets “exclusive rights” to both parties – Seol can’t see Yul, so Se-joo gets to openly date her, and since Yul can only communicate with Se-joo, Se-joo can selectively react to him or choose what to tell Seol. Such as when the three go on a date around town (Gwanghwamun), we know it’s mainly for Yul’s benefit, but Seol thinks Se-joo choice of a popular tourist route for a date is anything but romantic. Or later, when Se-joo “introduces” Seol to his invisible friend and is comically blasé about levitating coins and self-opening doors (juz think back to his initial shit-his-pants reaction when he realizes Yul is a ghost). Too bad this arrangement doesn’t last for long. Se-joo is clearly shocked (and possibly disappointed) that Seol now can see Yul as well.
The current time line love triangle is comedic (re: the Pavlovian Dogs analogy) compared to the 30s’ love triangle. Even the threats of Sang-mi and Tae-min pale in comparison to the risks Soo-hyun, Hwi-young and Yul face as Freedom Fighters in the 30s. I feel the love triangle then to be more swoony than the present. Imo, the alley kiss between Hwi-young and Soo-hyun is undoubtedly more exciting than the one Se-joo gave Seol in the hospital. Maybe the over-hanging possibilities of death or imprisonment give that added special urgency to their romantic relationships in the 30s. Carpe diem – to seize the day – very apt indeed.