Lee Seol is the deposed crown prince who was supposed to be killed, but DanOh had saved him back then. 

Lee Seol's parents were dead and he has been chased by the royal soldiers since then and currently he is called Speck of dust. 

Lee Seol might be: 

1. Kang San (good at martial arts, has a sword and hidden background)
2. Yoo Ha (who's a scholar & is hidden child of his father)
3. Shi Yeol (gambler, broke & cute behaved man)

I really want Kang San to be LS. I'm pretty sure Yoo Ha is not(But it's still 2 episodes, so not 100% sure). But whoever it is I just want the girl to end up with whoever is LS. ;D . LS's character was made stiff and upright personality in his kid versions, so it's hard to think of Shi Yeol as LS. 

I’m curious too.  I really think it has to be one of the three.  I am leaning towards  Yoo Ha.  

We all know now on episode3, Kang San is the watchman.  Yoo Ha, seems to be the illegitimate son of the  Crown Prince and the courtesan lady that runs the Bo something place. My bet truly is ShiYeol, because the dogs like him and made a hint that what if one of us the Lee Seol. Also, he's name Shi Yeol close to Lee Sell. That's my intake on these drama

 seonni:

If I know anything about historical romcom clichés, I'm 99.9% confident that Kang San is the deposed crown prince. There are just too many tropes that align with his character. I think the personalities and looks also match well with the younger versions, and also it makes the most sense for him to be Lee Seol with all we have been shown so far.

I don't think he is,  on episode 3, the head eunuchs already told Kang San either kill Lee Seal or put him on the throne.

 seonni:

That was not an order, but a demonstration of his own power, since he is the only one who can recognize him, he can either choose to denounce him for treachery to those who want him gone or confirm he is indeed the crown prince if he is to be reinstated again.

On episode 4, towards the end the prince is She Yeol.

After watching episode 6 I think Yoo ha is the deposed crown prince and kang san as the watchman. Or Kang san can be the crown prince who's also playing the role of watchman.

Six episodes done and I’m leaning more to Kang San as the deposed crown prince although Siyeol is kinda sus to me as well. But I think it’s really KS and him trying to guise as the watchman is part of their plan to overthrow the current king.