Paladins? Think Knights Templar. Part of a mixed adventuring group? I don't think so. However, don't be surprised to meet an army of them or a small group of them at least, on pilgrimage.
Players Handbook (page 22)
A paladin character is a fighter sub-class, but unlike normal fighters, all paladins must begin as lawful good in alignment (q.v.) and always remain lawful good or absolutely lose all of the special powers which are given to them. They have both fighting abilities and limited spell powers (at high level).
House Rule
You cannot be one as a player character. Simple.
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