Nature & Appearance
Familiars exist within the Spirit Realm, a world parallel to Midgard, existing in tandem with the world of mortals. Every mortal soul in Midgard has a familiar counterpart in the spirit realm. They are connected with someone's soul from the moment that person is born, and they share their "master's" exact personality and feelings (though they do not always match their "master's" sex), and the familiar can always hear the thoughts of their counterpart in the mortal realm, even when they are not present.
The familiar is not named or commanded by its mortal master; they have their own names and follow their own orders. The familiar and their "master" (a rather inaccurate term, but one most often used, nonetheless) are equals, and the familiar chooses whether or not to obey and whether it will come when summoned.
All familiars take the form of an ordinary animal, not a magical creature or anything humanoid, that reflects elements of the personality found in their "masters," or their counterparts in the mortal world. Thus, familiars range in size and shape from the smallest beetle to the largest tiger, depending upon both where their "master" is from (familiars only take the shape of an animal native to the region of their mortal counterpart; for instance, a Nordling would never have a tiger familiar), and depending upon the behavior and character of their "master."
The wisest of shamans can read an individual and know their familiar spirit well before that familiar is summoned, if it ever is. Knowing someone's familiar spirit is essentially receiving a look into their true character, the very makeup of their soul, and it can be unmatched in terms of finding someone's true nature. For instance, if someone who seems trustworthy has a vulture for a familiar, it may be time to reassess their character. However, it is also true that many familiars are terribly misjudged, and thus one should only trust a knowledgeable shaman to interpret whether someone having a rat familiar means they are intelligent and enduring, or if they are a cowardly betrayer, or possibly both.
A familiar spirit's abilities are, essentially, the same as any ordinary animal of their type, but with incredible and uncanny intelligence on the same level as their "master." However, the bond between the familiar and his or her "master" allows them to communicate in ways no other creatures can. The "master" and the familiar can communicate to each other through their souls, not unlike a form of telepathy, and they can hear each other's thoughts. The familiar cannot communicate with anyone else in this way, however, nor can they speak aloud.
In order to summon one's familiar, the person must first enter the spirit realm using Spiritual Magic, whether through their own means or with the aid of a shaman or druid. The person need only request, in his or her mind, for their familiar to come to them. If the familiar is willing, they will appear, and the two must make an agreement regarding whether the familiar is willing to be summoned to the mortal realm. However, not all familiars come when called, particularly if they dislike the manner in which their "master" addresses them, and often it can take several visits and a considerable amount of coaxing for one's familiar to answer their call (this also varies considerably based on the individual's personality). If the familiar agrees to be summoned, the "master" will find their familiar present with them in the mortal realm when they exit their trance.
When a familiar spirit is killed in the mortal realm, they are cast back into the spirit realm until they recover their strength, at which point they can (if they are willing) be resummoned again. However, when a familiar is "killed" (temporarily banished back to the spirit realm) in Midgard, their "master" also suffers a terrible blow to their own soul, considerably weakening them for so long as the familiar is still recovering in the spirit world.
Many individuals who summon their familiar spirit enchant certain items to become anchors for their familiar. Most anything can be made into a familiar's anchor. Using the anchor, the "master" can summon or unsummon their familiar spirit at will - though, of course, the familiar still needs to be willing to answer that call. Using anchors allows the summoner to dismiss or summon their familiar as convenient, in hopes of avoiding their familiar being "killed" in the mortal realm.