What TV drama character has the most visible change throughout the series?

please mention the name of the character.

thanks in advance.

Burning Ice - male lead. That development was for me one of the most extreme ones. He truly did a 180 degree turn on who he was and what morals he stood by.

 AH :

By "visible", do you mean physical changes? Or changes in their behaviour / personality / character?

changes in their behaviour / personality / character

Empress Ki    (Yuan Emperor Hui Zhong / Ta Hwan)

Anna    (  Lee Yoo Mi / Lee Anna  )

Queen Seon Duk    ( Queen Seon Duk  )

Mr. Queen    (the queen)

My Golden Life - Seo Jian +1 (IMO Shin Hye Sun plays the most pitiful character I've seen in kdrama)

Very subjective though. Long historical sageuk dramas such as Empress Ki, Queen Seon Duk, Jewel in the Palace, Dong Yi, usually have (FL) characters go through a lot of change during their life.