Q:

Korean cheerleaders

I notice that at Korean sporting events, groups of fans will set a called cheer. So, what happens is a girl shouts out in cadence and calls for a cheer, then the fan group answers altogether with a cheer.
What is the cheer leader girl called?
EXAMPLE:
At 6:05 of the following video, you can hear examples of the cheers. First you hear the cheer for the first contestant ("Jin Air FIGHTING !!!"), then a second similar cheer is given by the fans of the opposing team ("SKT FIGHTING !!!").
https://www.youtube.com/watch?v=njxx0yCNqv8

A:

The cheer you've described is called 선창(先唱) -- literally a "leading call."

진에어 하나 둘 셋!
진에어 화이팅!

Quick googling gives me two kinds of usage. Properly speaking, only the first line ("진에어 하나 둘 셋!") should be called the 선창 and the latter ("진에어 화이팅!") the 후창(後唱), the "following call." But colloquially, both lines altogether could be called 선창 or 선창 응원 by some.
For the one who did the 선창, I don't think there is a term especially for the role. But probably I would call her 선창 하셨던 분 ("the one who did the leading call") or just 선창.

