1. The character Marquis Halim Ondore IV in the video game Final Fantasy XII. He spread several falsehoods in the game, prompting a party member to exclaim, "Don't believe Ondore's lies!" Soon 4chan's /v/ discovered that Ondore in fact tells nothing BUT lies.
2. Has come to mean any liar in general.
Whether encountering "the" Ondore or any random ondore: DO NOT BELIEVE HIS LIES.
I think I'm going to buy that Phoenix Wright game. Ondore said it was terrible, so it must be great.
My girlfriend says she's not cheating on me, but she's something of an ondore, so I don't believe her.