Found: Prester John

Doubt not. The real Prester John is revealed! I cite that reliable source: the Fantastic Four, where he first appeared in Issue 54.

This being the Internet, there is of course a full indexed chronology produced by scary obsessives.

And for those who aren't going to read the whole of Baudolino,

• the interesting love scene is on pages 441-442 of my edition, chapter 34.
• The set-up for the whodunnit is in chapter 25 on p.305
  
• Who killed Frederick (and who has the Grail) is revealed in chapter 38.
  
• Well, actually in the next chapter, on p. 511