Chun Woo from The Breaker may act like he doesn't care about Shi Woon, but if you touch his disciple you're dead. Talking shit about his master's death will also get him very mad. Also, seeing Shiho badly hurt makes Chun Woo one-hit that him.
Eventually, Shi Woon gets his own: anyone threatening his Love Interest Sae Hee.
For Elder Kwong anything related to Sun Woo clan.
Don't ever say the vice-principal has a "shitty car" or damage his car.
Yureka: Referring to the real Yureka as "Fake Yureka".
Mentioning Lotto's height deficiencies.
Seunghee, from Zippy Ziggy, really hates perverts and being called "old".
Mispronouncing Duckchill's name.
From Magician threatening Natasha will send the normally stoic immortal magician Edermask into a lightning fuelled rage.
From Dorothy of Oz going near Mara with bad intentions is a sure way to upset Abee.
Mara doesn't like being called "Dorothy".
Yue's button, from Witch Hunter, is hurting Xing, or even mentioning it.
Haji from Hot Blooded Woman really hates being called a puppy. Since her full name, "Kang Haji", sounds like the Korean word for "puppy", she really hates it when people call her by her full name.
Harming anybody weak or genuinely good is Rai's button from Noblesse.
Mentioning Munsu's former lover or best friend in a negative light, or praise Aji Tae, is Munsu's button from Shin Angyo Onshi.