Besides Lionheart, I never really bother to level any of them up.

As for how I build them, I normally leave them more or less default:

Ming-Wu: I steal his staves to sell them for quick cash. I let him cast his spells, but he's already powerful enough so I don't normally bother leveling up any of his attributes.

Joseph: He just hits things. He doesn't stay long.

Gordon: I give him dual spears.

Leila: I give her daggers and level up her Thunder magic.

Richard: Two spears, if I recall, you find some nice spears for the duration of his stay.

Lionheart: This varies. Usually I'll give him spears or swords, or even axes. I may also give him some helpful magic like Cure and such.