I am looking for dramas where the main lead or the secondary lead is someone who is very hateable but changes for better during the drama.

Some examples that I watched and loved & definitelly recommend to others:

Lee Kang-To - Bridal Mask

Lee Joon - Chocolate

please mention the name of the character.

many thanks for considering my request.

The male lead in The Matchmakers was egotistical, very dislikeable, and angry due to the twist of fate that had made him the king's widowed son-in-law on his wedding day, by law in perpetual mourning. However, at episode 10 they completely rewrote his character and background to make him far more sympathetic. The drama was popular and is certainly watchable, but this threw me off for the remainder of the episodes. (I was happy when he was becoming nicer.)