• Make them unfair, especially if it’s towards the main characters. Bonus points if they know they’re being unfair and dismiss it.
• Make them a bigot.
• Give them a position of power.
• Do not give them a zany personality. Anything can be forgiven for a character with a wacky personality.
• Let them get their way at important points instead of having the main characters constantly triumph against them.

Umbridge fits most, if not all, of these points.