The concept of the magic-user's familiar has a long pedigree in fantasy tales and games. In an animal fantasy campaign, the Familiar can become a protagonist in her own right. While she largely plays a support role for a Magic-User, the Familiar is endowed with certain unique abilites of her own that make her a viable character option... especially if the Magic-User in question is an NPC.
This niche is optional, and intended for use in campaigns that feature "real"magic, such as a standard classic fantasy setting, or a modern fantasy setting. It has no place in "reality-lite" campaigns where the supernatural is a subtle influence if it exists at all.
Magic-Users are notoriously vulnerable at the starts of their adventuring careers, and often call upon arcane forces to protect themselves. Some spells or rituals allow Magic-Users to call an animal into their service, with whom they share a magical bond that benefits both parties. These animals, called Familiars, then embark on a life of adventure with their Magic-User partner. And some learn from their "master," then go on to earn their independence.
Niche Ability: When you take Familiar as your niche, you benefit from a handful of special abilities rooted in your bond to the Magic-User who summoned you.
Your mystical bond with a Magic-User allows you to add her maximum possible hit points to your own total, giving you both a better chance at survival. However, if your Magic-User is ever killed, you must permanently subtract these hit points from your own maximum possible total, which may lead to your own death.
Second, you share an empathic link with your Magic-User, and are always aware of her general location within 200 feet. You may share your perceptions with her, and she with you, giving each of you a form of remote sensing, and you may speak to each other in your native tongues with perfect understanding. You also benefit from any spells your Magic-User casts on herself.
Third, you gain a bonus Species Trait from your bond: a +1 bonus to any single lore outside your own (reflecting the fact that most Magic-Users are human, and you are thus sharing in their versatility). If you become the Familiar of an Elf, you instead gain immunity to the touch of ghouls. If the Magic-User who called you is a member of some other species, you gain a bonus Species Trait from them based on the BM's judgment. (NOTE: In some settings, such as the Trucewood Vale, it is possible for animal Magic-Users to have longpaw familiars!)
Finally, you receive a +1 bonus on all saving throws so long as you are in service to your Magic-User.
Beyond 0 level, this bond grants you further advantages.
At 1st level, a you become an arcane dabbler. With a successful lore check, you can activate magical items normally only usable by Magic-Users as though you were a Magic-User of equivalent level.
At 2nd level, you become capable of delivering touch spells from your Magic-User to targets normally beyond her reach, as though you are an extension of her body. And with a successful Familiar lore check on your part, the activation of this spell no longer depends on the will of your Magic-User. She can cast another spell before you deliver the one you are carrying, and you are capable of changing the intended target to one of your own choosing.
And at 6th level, you become capable of storing extra spells within your mind, that your Magic-User can call upon as though she had remembered or prepared them for the day. The choice of which spells are stored in this way belongs to your Magic-User. However, you're also able to activate these spells yourself, with a successful Familiar lore check! You can store a number of spell levels equal to your Familiar level. Thus, at 6th level, you could hold six 1st-level spells, three 2nd-level spells, two 3rd-level spells, or any other combination of levels that adds up to your total Familiar level (for instance, four 1st-level spells and one 2nd-level spell).
At 10th level or any time following, after years of loyal service, you have the option of gaining your independence from your Magic-User. If you part ways amicably, you lose all of the above abilities except the extra hit points, bonus on saves, and arcane dabbling, and gain the spell-casting abilities of a Magic-User equal to half your level. You may then continue to advance in spell-casting abilities by level as though you were a Magic-User yourself... even being able to summon a Familiar of your very own!
Other Abilities: Add your Familiar level (plus your Familiar niche die result, if applicable) to all lore checks when attempting to intuit the function of magical items, recall or remember ancient mysteries, magic traditions, or cryptic phrases, or to deduce what kind of spell a Magic-User is about to cast by observing her gestures.
Saving Throw: Spells
(+9). This increases to Average (+6) at 2nd total level, Tough (+3) at
5th total level, and Challenging (+0) at 8th total level.