The Life spell: Sprite Guardian doesn't work good. When my life character summoned the sprite she was down to half health and the sprite just kept attacking even though she was dieing! I think the sprite should heal when someone needs it and then, when there is no one to heal it should attack.
I use my sprite guardian a lot, and I think generally she makes excellent choices. With a modest amount of damage she will fairy, with a lot of damage she will satyr. Every once in a while she just keeps attacking, but more often she heals.
The Sprite does get absent-minded once in a while; it's a random thing. For the most part, she'll heal whoever needs to be healed, including herself. However, it's not something to be depended on.
The Sprite makes it possible for Life wizards to solo many tough battles, as she will usually heal you if you get low. She can be a pest sometimes, as she'll attack an enemy with her Imp spell.... and set off the Life Trap and Feint you've put on the enemy. She tends to always attack the same enemy until it's defeated, so you can put your traps on other enemies.
If you need better protection, you have to hire a Henchman. However, even the Henchman will sometimes not heal you when it's needed.
I have a life wizard (My best one!) She has sprite guardian and I use it well. I usally use the Sprite (whitch I called Pixie Lott). She is very useful and heals me, herself and she attacks! So I dont know whats happening with yours! :D