The backstory gets a tad creative with the world outside of Sleangate, so feel free to veto it if needs be.
Karis was not born in Sleangate, nor had he any interest in its affairs. In fact, he was born the far side of the Krakendepth sea, in a small but ancient kingdom known as Oshain. The first thing recalled when it is mentioned is its heat - its terrain ranging from jungle to desert. The second is its wildlife - even the ordinary creatures, such as the deserts giant scorpions or the jungles fierce hunting cats, have oft been mistaken for monsters.
Karis himself was born a prince to this kingdom. His father ruled from the sandstone palace, a young man, and the children - for Karis had both a brother and a sister - were left much to their own devices. His sister was older by a year, and his brother by another. But all was not as well as it appeared. An unhealthy affection grew between Karis's sister and brother, and the brother began to grow discontent watching their father stay healthy. Lust for power and love tore at the brother, and on the feast for Karis's 21st birthday (and to celebrate the opening of a new menagerie, attended by none other than a Miss Tannerson), their father collapsed, poisoned.
Karis, enraged, leapt across the table, and hacked his brother to pieces with the carving knife he held. However, the affairs of the family had been kept within the family, and Karis's mother had died at his birth - leaving himself and his brother's lover the only witnesses to his innocence. Karis was dragged to a cell, and lay there for days until a trusting servant released him for a quick stroll. He fled through the city to the port, where he met with an old childhood friend, who agreed to take him out of the city by ship.
The Gryphon sailed that evening, bound for a port a short sail down the coast. It arrived the following evening to a red and black sky, the port in flames. A proud silhouette of a ship rode out to greet them, a manticore sculpted into its prow. As the Gryphon and the Manticore fought, Karis watched from the prow, cursing in every tongue he knew. The Gryphon was soon boarded, and its captain surrendered, with Karis and the crew gathered in the centre.
The Manticore's captain, a pirate lord from the island haven of Naucisca, ordered the crew killed, with the prince and the Gryphon's captain left for last. However, he must have seen something in Karis's eyes, for he made him an offer - kill the Gryphon's captain, and he would be free to go. He saw no choice, and soon the captain's blood joined his crews on the deck. But Karis did not stop there. The Manticore's captain too, soon lay dead, along with a number of his crew. Those that remained unanimously voted to instate Karis as their new captain.
Karis and his newfound crew haunted those seas for the best part of three years, and he found out that his sister had been given queenhood. She took a naval captain for a king soon after, who went to great lengths to hunt the pirates and their princes. He died on Karis's blade, and Karis was rechristened by the Pirate Lords - the called him Kas, the Red Manticore. But soon the sister married once more, to a pirate lord himself, and the coast of Oshain was declared off limits.
In disgust, Kas left the Oshain coast, and left the Isles of Naucisca, and took his ship off into the Krakendepth, sailing until he reached the far shore. A passing merchant spoke, under duress, of monsters, and the half remembered name of Tannerson. With no other direction, Kas sailed for Sleansgate, and two miles out, left his ship to his first mate, and arrived on its shores in a small rowboat - only a manticore claw on a bootlace to remind him of those past years.
A foreigner in a strange land, Kas "Seablown" went almost immediately to search out Tannerson, and only hoped she might remember him.