It has been said that there is nothing new under the sun.

On the other hand, if a lawyer can recognize a character as his client's work it's plagiarism.

The question you should be asking is, "How much of this character did I create?" If the answer is less than 100%, then you need to acknowledge that. With the permission of the creator you can use it. If the character is wholly your own, but was used in a game, then you can use the character but not elements of the game.

Let's say I created Captain Roger for a game of Sea Dogs. As long as I avoid any reference to the governor of Highrock or other elements of the game I'm good. If I use elements of the RPG story, however, I have crossed the line.