NotJabba: All hearsay, I guess, as I've never played using Keldorn so far. I know any good character will leave you if you have too low a reputation, but I've been led to believe that Keldorn will also ditch you if you ever turn into the Slayer or if you side with Bodhi in the guild war (which I generally do, as I have a much more visceral dislike for organized crime than I do for fictitiously evil creatures, and I know I get to face Bodhi either way). I don't know of any other NPC who behaves that way.
Correct on both counts. Here are the party critical Keldorn events (not counting reputation) to watch out for:
Keldorn will leave the party if you...
** Attack Sir William
** Side with Cowled Wizards against Valygar
** Rob temple of Lathander
** Refuse to help Renfeld
** Side with Bodhi
Keldorn will leave and attack the player if you...
** Attack Valygar
** Attack Adalon
** Give Adalon's eggs to the demon in Ust Natha
** Attack Drizzt
** Attack Svirfneblin village in the Underdark
** Turn into Slayer (UNLESS Mazzy is present, who takes precedence)