Hello! I've started to really enjoy watching dramas where one guy ''serves'' another guy and I wonder if there's anymore dramas with characters like that, like one's the servant/assistant/right-hand man & the other one's the boss/master/leader. Kind of like the relationship between Yi Bang Won & Muhyul in six flying dragons or Heung Sam & Praying Mantis in Last.

If you know any characters like those, no matter if it's historical or present, I'll take everything! :D

Thanks!!

Jewel in the Palace (Dae Jang Geum) but the master/disciple relationships are between females.
Incomplete Life (Misaeng) has a boss/worker relationship
Hong Gil Dong has a master/student relationship.

The Royal Gambler (It has loads of these kinds of relationships)
Arang and the Magistrate
City Hunter
Faith (General and King)
Scholar Who Walks the Night (2 really good relationships like this)
Shopping King Louis
Iljimae (lots of these relationships)

The Moon that Embraces the Sun
The Three Musketeers
The Disguiser--c drama
Arg...you seems to have watch many things I wanted to recommend, like for example 'Cruel City'

If you don't mind melodrama then 'A Man's Story' has Heung Sam & Praying Mantis kind of relationship between Chae Do Woo and his bodyguard.

If you don't mind a movie instead of series then the The New World also fits.

If you don't mind it not being k-drama then Nirvana in Fire has several such relationships.
I second Cruel City! 

Love in the Moonlight. 
Thought of another one you don't have on your list: 'Please Come Back, Mister'

It's a comedy unlike the other ones but  the relationship between Han Hong Nan /Gi Tak and Choi Seung Jae is very much like that of Heung Sam & Praying Mantis and it was mainly then who made me enjoy this show.