History
During his younger years, Kasai was always known for his excellent proficiency in Firebending. At the young age of three his father assigned him to a master and forced him to learn the Bending Arts of Fire. It was completely unheard of for anyone of such a young age to Bend, let alone walk. Yet, he did it. By the age of four he was performing feats that most couldn't even do at age ten, which just so happens to be when most people actually take up Bending as a profession, if they have decided that is. For many years he trained consistently, being forced to continue despite not wanting to. Then, after so long, he finally got tired of it. At this point, though, he was much more powerful than his Master, no matter what he believed. Using the bending his Master had taught him, and much he had taught himself, he escape from his family's grasp and set out to find a life of his own.
For an entire decade he traveled the lands of the Fire Nation, searching for his own place in the world and not one his parents had decided for him. It was his time to make his own choices. Some happened to be wrong, some good. For instance, an example of a wrong, is when he accidentally pissed off an entire Royal Armada of Fire Nation ships, forcing him to fend for himself for weeks before he could come out of hiding. Then, there are the good parts. Those stories of the good can be left for another day though. It's time to get to the most important part. Now, after two decades of travel, he has come across true love. Starla Jinxun, just the average civilian woman anyone would find in a run down town. For many months they dated, then eventually got married. Together they conceived three children, two males and one female. Though, just like everything else, nothing lasts forever.
A Fire Nation Imperial Soldier found himself in the town he had settled himself in. This wasn't just any Imperial Solder though. It was one who was feared for being the most vile and cruel of the entire military. Well, that feared Imperial fancied his own, and decided to take things into his own hands. He pestered Starla, insisting that she come with him to Capital City for some "Fun", but when she refused for a third time he lost it. In cold blood, while Kasai was not around, he murdered Starla and the three children then made his escape. Kasai, after returning and discovering the news, was completely infuriated and broke down emotionally. For two weeks he weeped for his fallen family. Then, after those two weeks, he resolved himself to become as strong as he could possibly be to prevent this from ever happening, and to assert his revenge.
With the loss of his entire family still fresh in his mind he set out to train himself, repeating training processes thousands upon thousands of times until they were absolutely perfected, or at least as close as they will ever be to perfect. Knowing things might get out of hand, his intense training and all, he made way of a remote mountain area, an area where few man dare approach. There he continued his training for two decades, completely isolated from the outside world. However, when he did make his appearance, he was completely overtaken by anger and sadness. He was fated to never exact his revenge, for the Imperial Soldier had died in battle many years back. It was a truly sad day for him indeed, decades of work put to waste in an instant.
In sadness he returned to his mountain home, isolating himself once more. This time though, he ran into something unexpected. Perched just outside his house was a Giant Fire Hawk, a species believed to be thought to have gone extinct over a thousand years ago. Yet, it was standing right in front of him, festering at his own feathers. Then, it noticed him. It didn't fly away, but instead it moved closer to Kasai. Once close enough he could feel the heat radiating off the magnificent bird, the sheer beauty of the creature overwhelming him. The most surprising part though, was that it rubbed up against him. It seemed as if it had taken a liking to him.
Now, with a new companion to keep him company, he continued what he was doing previously. He continued to hone his Firebending for decades to come, even working together with the Giant Fire Hawk for great cooperation. Several more decades pass and he finds himself to be at the very pinnacle of his abilities, no longer to progress any further. So, rather than spending the rest of his life in isolation, he made his way to "Republic City", a place he had heard about. It was supposed to be a peaceful place, but he will have to find that out by himself. Riding to the city on his Giant Fire Hawk he acted like a drunk old man, the species of bird so long forgotten that it wouldn't rise much suspicion from the younger generation. To them it would be just any creature found in the wilderness. Though, to his facade, it was his beloved Chicken.