The gadfly is a character who often says things they don't necessarily believe in order to get a reaction. Maybe they have a hard time really expressing themselves. Or it could be they just like to watch others get flustered, confused or angry.
Usually they're not really bad people. They can be quite amusing as long as they're not going after you.
Which they probably won't, because the target is usually limited to characters the gadfly knows well or react strongly.
Compare The Tease
for overtly sexual examples. Compare Troll
for as a Real Life example for when this is a person on an internet forum. See also I Shall Taunt You
The name comes from the term social gadfly. The toper was Socrates, who said that like a gadfly he could be easily swatted, but that a government who does such thing pays too heavy a price. The gadfly would help improve politics by raising unpopular, controversial viewpoints for discussion.
This didn't work out so well for him. And now it generally refers to someone who simply causes trouble.
Needs a better description.
- Albireo in Mahou Sensei Negima, who is the only character that can reliably get a rise out of Evangeline. He's described as her 'natural enemy.'
- Yuichi in Kanon is fond of making bizarre and outrageous claims while maintaining a completely serious atmosphere in order to confuse all his love interests. Few manage to catch on and also recognize when he is and isn't serious.
- Half the time when Koizumi and Kyon talk, Koizumi will explain some theory to explain Haruhi and Kyon will start getting very involved in the conversation and thinking about it deeply. At which point he says that Just kidding, I made the whole thing up.
- Asuka Langley, due to her huge insecurities. Basically a big cry for attention in her case. Is also a jerkass.
- Hikaru and Kaoru in Ouran Highschool Host Club generally amuse themselves by baiting Tamaki and Haruhi. Tamaki is a bit of a buttmonkey, but Haruhi can hold her own. Often unintentionally.
- In Robert Heinlein's Rocket Ship Galileo, Dr. Cargraves does this when debating with his three teenage apprentices, to get them to question their own assumptions and realize the importance of being able to prove their assertions. His contrary position is that the Moon might not have a "far side", because no one has ever seen it (the book was written before spacecraft were sent to photograph the far side).