Alf is a magical beast that can acquier human form if I remember correctly. She is Fate's loyal guardian.

Yuuno on the other hand is a magic user who lost his powers on first season (or depleted them, don't remember) so he took that shape. Why did he took it once more on Season 2? (don't remember if he did, but if he did... it was to spy Nanoha... LOL!!)