cracks his knuckles
Healers can:
-attack and chase any undead (vampire, or lich as well) to the ends of this world.
-defend against any assailers who would dare touch a healer (how aggressive you are in defending depends solely on the ethos and roleplay of the healer)
Caballed:
Tribunal-find and apprehend criminals all you want, but depending on your ethos, you may want to apprehend those that come in the cities only. Most of these Tribunal healers should tend to be more regal and diplomatic in what they do.
Knight-attack anyone who attacks the castle. If that pesky warmaster thief wants to toy with the knight defender, I believe you have the go ahead to introduce him to Chayesh. Healers in Knight are tricky as they can go several ways. Like Crisnae you can get outcasted and attack all evils/nexus. Or like Ecasio, you can attack only specific characters. Ecasio was a bit awkward because he borderlined the outcast line. However, as long as you have a good explanation for every corpse that litters the lands, have fun.
Healers are great for learning the lands, equipment, and basic running skills. And by running, I do not mean gating to the farthest mob you can think of. Put up all your spells and allow that ax-wielding ogre to chase you across the dead seas. I certainly learned a lot.
Necro: Ecasio attacked anything that was undead. Zombies are no exception. Just make sure that if you kill the necro, it was only because he kept attacking you after you fled at least once. This would remove any doubt that you were aiming to kill the necromancer instead of his zombies.