Some people think Seunghyun loved Jaeil romantically, while others believe his feelings were purely platonic—more like admiration, loyalty, or even idolization.
The story has never clearly stated what kind of love Seunghyun felt for Jaeil. After all, he was an orphan who spent most of his life serving Jaeil after being taken in at around eleven years old. Jaeil was basically the center of his world.
Seunghyun would get shy whenever Jaeil complimented him, which was enough to make many fans ship them back then. Since Pearl Boy was such a popular series with a large fandom, these two gained a lot of shippers, just like the twins did.
Because the author seemed to hint that the twins had a sexual relationship—which is probably true, considering many fans believe the author plans to write their story after ITD ends—some readers also started wondering whether Seunghyun's feelings for Jaeil might have been romantic as well. Since there was already an incest couple in the story, many fans felt that a pseudo-incest relationship wouldn't be out of place.
On top of that, Seunghyun was incredibly attached to Jaeil. He was Jaeil's loyal right-hand man, had no close friends, and didn't seem to have meaningful relationships with anyone besides Jaeil and his two sisters, who likely lived far away.
That kind of dynamic is actually quite popular in the BL community these days: a younger top who grows up under the care of an older bottom and becomes deeply attached to him.
So when the author had Jaeil marry a woman—Dooshik's sister—a lot of fans were furious. Many blamed both the author and the female character herself. Then, when Yuta was revealed to be Seunghyun's love interest, some of that backlash was redirected toward Yuta as well.
To this day, Yuta still receives hate from certain parts of the fandom because he effectively became the obstacle that prevented the Seunghyun–Jaeil ship from becoming canon.
Wait what’s the context behind these characters in the og story. I don’t wanna read it so.. what’s the relationship between the retired boss and the mc. I don’t wanna seem stupid for speculating but some authors are mad weird so you never know