The title really says it all.

Is there a way to put a page for character/enemy bios in the in-game menu in RPGmaker XP? And if so, how can I make it so that a character/enemy is only put in there after the player encounters them? Is there a script or trick to this?

http://forum.chaos-project.com/index.php/topic,6783.0.html

Check out the pinned script list from the script submissions section:
http://www.rpgrevolution.com/forums/index....showtopic=25015

There is a journal system section with about 11 journal scripts. I think blizzard's is what you are looking for. Cold's is very nice as well.