Kotetsu is an excellent master swordsmith, having forged the legendary Nidai Kitetsu which is powerful enough to be classified as one of the 21 Great Grade swords. Through unknown means, he was also able to place curses on his swords, which certain individuals, such as Zoro, can even sense.
Kotetsu lived sometime long ago. Sometime during his life, he crafted the sword Nidai Kitetsu. He also had children that continued his lineage.
Kotetsu's name might come from Nagasone Kotetsu, a famous japanese swordsmith from the Edo Period.