The Nameless One is the character you play in Planescape: Torment;
he's a battle-scarred amnesiac man who seems to be immortal.
The catch to his immortality is that each time he "dies", he
wakes up with no memories of what happened to him.
This includes memories of his skills. In his long life, the Nameless One has been a 24th level cleric, warrior, thief, and mage; The catch is that he loses the powers when he dies.
In the game, the Nameless One can choose to learn new skills
by getting training. He can also talk to characters in the
game to help him remember his skills. Finally, you can train
to get new skills. However, you may have to pay for such training,
and you don't always pay in gold.