Sidekick/companion

Need a grouping of all the Sidekick/companions that are possible:

Obtained from skill/item.

Naturalist:
Wolf
Water buffalo

Psion:
Wind Warrior

Obtained from dropped item.

Blues musician
Fledgling elemental hero
Strobe drone

Obtained from a "source".

Plaid couch:
Boxing coach
Crow
Free dummy
Mafia informer
Old friend
Smart-aleck kid