What word

[Martin Jones]

What word describes a character in a play or book whose name is in the title

[Quizmonster]

Eponymous. For example, Oliver Twist is the eponymous hero of the novel 'Oliver Twist'.

[mycatis]

Damn damn damn, I knew that one, too slow :)

[WaldoMcFroog]

Or, it could be 'Titular' - I think this would distinguish it from an eponymous character as described by QM in his example. Take 'The Picture of Dorian Grey' - since 'Dorian Grey' is not the title but his name appears in the title, titualr would, I think, be more appropriate. I'm sure I'll be corrected if this is wrong!