The Shaman's Call Spirit Companion feature in Player's Handbook 2 (page 120) says the following about the Shaman's spirit companion:
The spirit can be targeted by a melee or ranged attacks, although it lacks hit points. If a single melee or ranged attack deals damage to the spirit equal to 10 + one-half your level or higher, the spirit disappears, and you take damage equal to 5 + one-half your level. Otherwise, the spirit is unaffected by the attack.
This makes it clear that the spirit companion can be attacked, but what are its defenses?