A spirit companion is a class feature of the shaman class. It is a conjuration effect, so in addition to the rules cited under call spirit companion, it also utilizes rules that pertain to the conjuration keyword (PH2, pg 220). Since I'd just started playing one, I wanted to figure out all the mechanical bits that went into it, and realized that others might have similar trouble with it.
It takes a minor action to summon a spirit companion, which appears within 20 squares of you. It lasts until you are dropped, or dismiss it as a minor action. As a conjuration effect, enemies cannot pass through it.
If you take a move action, it can also move up to your Speed. I have houseruled this to allow it to make a shift if you want, as a ranger can with animal companions.
It can only be targeted by melee or ranged attacks. This seems very specific, discounting area bursts and close blast attacks (a ranged attack is not the same as an area burst despite both being ranged). This means that your spirit is effectively immune to a dragon's breath weapon.
If it takes damage equal to 10 + one-half your level it disappears, but you can summon it again on your next turn. You also take damage equal to 5 + one-half your level. It is otherwise unaffected by the attack. As a conjuration, the spirit uses your Defenses and is not affected by conditions or ongoing damage.
Also if you do not have line of sight to it at the end of your turn, the effect ends.
Unlike most attacks with a conjuration, if you use a power with the Spirit keyword, you draw both line of effect and line of sight from the spirit, not yourself. This is an exception to the normal rule in that you must have line of sight from yourself.