Well, in a familiars case they use what the animal would have normally in skills and feats, they don't gain new feats nor skills as you level, but their HD is either equal to thier base animal or masters whichever is higher, but hp is always half their masters. And even though the skills don't progress, they get their base animal's skills or their masters whichever is higher, which is easy to abuse for always having someone to assist you on a check.