I am told that the only Paladin NPC on the game is Keldorn.
Is this true? If not, who else is there?

Yes Keldorn is the only paladin NPC that can join your party

<font color="gold">Keldorn is the only Paladin-NPC that can join you, yes.
But you can also get Anomen. He isnt a Paladin, he's a Fighter/Priest, and can reach Knightship in the Order. But he still aint a real Paladin. ;) </font>

Mazzy is Paladinish, but she can't use Carsomyr, soo...

if you're good enough, you can edit Carsomyr to also be usable by halflings and straight Fighters. Just don't get carried away and start using it with your evil barbarian, now... try to keep at least a little of the game's integrity...

Besides, Keldorn is good. He is an interesting character and a strong NPC (with the Bracers of Dexterity, of course). Just boost his strength and dexterity and give him Carsomyr and you're golden. don't act like he's the *only* paladin. Paladins are rare, and the fact that a paladin is provided at all should not be taken for granted.