The Joker vs. Flame Boy.

Most people don't know the true story behind the rivalry between Flame Boy and The Joker, humanity thinks that reason behind this is that Flame Boy is Batman's 2nd cousin on his mothers side, well humanity is wrong The Joker has hated Flame Boy as far back as Princess Diana's paper anniversary. But why, where, how? All these questions will be answered in due time, but first we must deduce who these two characters are. Let's start with Flame Boy, a young man growing up in East Sussex, Fla Mebo was known for his Couldn't Give a Crap attitude but that very same attitude was his downfall. You see Fla was what one would call a Pyromaniac, he enjoyed playing with pirates, one day Blackbeard and Fla where chilling out, and Fla decided to start tickling Mr. Beard, one thing lead to another and the cigar Blackbeard was smoking accidentally lit Fla's gasoline based hair gel on fire. He ran about screaming which, in Blackbeard's eyes, was hilarious. Blackbeard came up with the name Flame Boy, wait a minute if you add a y into Fla Mebo you get Flame Boy, sorry I side tracked there. The Joker on the other hand is the arch enemy of Batman. Now we know the story of both The Joker and Flame Boy, I can now finish the main story, as young children they were the best of friends until one day The Joker took Flame Boy's building blocks, which didn't make him happy, so shit got serious and a fight broke out, both of their parents didn't stop it, the bet on it. The fight went on for years until one day they forgot what it was they were fighting about and just stopped…

The Joker one day remembered and text Flame Boy about but he never heard back, and that is why there us an Ultimate Rivalry between the two.

TL;DR they hate each other