Invoker GM's Notes

For the DM one of the most notable things about the invoker is not the class itself but the nature spirits an invoker bonds with. It is important to decide what the spirits are and how they interact with your world.

The spirit companion’s type is fey, and fey courts with kings, queens, knights and courtiers are a common element in medieval fantasy you (and I) can easily build of. On the other hand you can go for more wild spirits of nature, with more dispersed or even animalistic social structures.
Plants and Elementals are also options for nature spirits, either as replacements for fey or alongside them.

If you need more fey you can do this by adding classes or with templates. Cavaliers are great for noble fey knights, bards can be both entertainers and scheming nobility. Druids take the place of mortal clergy, and druids trained in a fey court could make for an interesting contrast to the more typical wilderness hermits. For templates take a look at Ultimate Intrigue and Ultimate Wilderness.
For templates good options are Fey Creature from Bestiary 3, Fey Animal from Lands of the Linnorn Kings and Fey Touched from Legacy of the First World. Green Ronin’s Advanced Bestiary has elemental creature templates and the Green Warden template for elemental and plant spirits respectively.
Interesting third-party products are Forest Kings Compendium, which is a collection of additional material for the Kingmaker AP. Dark Fey from Kobold Press which is a collection of evil fey and Monster Enhancement – Enhanced Fey by Purple Duck Games.

Otzeta-jha is is widely known as one of the most spectacular and ancient forests in existence. Many unique animals and fey make their home there. Only the druids of the temple at the heart of the woods know that the entire forest can rise up one being. (Green Warden Plant-Imbued Daikaiju Turtle).

The fey of desert lands are rarely as organized and united as those of more verdant lands. The courts of desert fey rove the sands, often warring with fellow fey and humanoid tribes alike. But all respect the knowledge and sanctity of Forsees, a simurgh seer who nests amidst the rocks at the center of the desert.

Mist Dog    CR 3
Fey Blink Dog
XP 800
CN Medium Fey
Init +4; Senses darkvision 60 ft., low-light vision, scent; Perception +7
Defense
AC 15, touch 14, flat-footed 14 (+4 Dex, +1 natural)
hp 22 (3d10+6)
Fort +5, Ref +5, Will +4; +4 vs. mind-affecting effects
Defensive Abilities; DR 5/cold iron; Resist cold 10, electricity 10
Offense
Speed 40 ft., fly 60 ft. (good)
Melee bite +5 (1d6)
Spell-like abilities (CL 7th; concentration +7)
    Constant—blink
    At will—quickened dimension door (self only)
    3/day—dancing lights
    1/day—entangle (DC 11), faerie fire, glitterdust (DC 12)
Statistics
Str 10, Dex 19, Con 14, Int 12, Wis 13, Cha 11
Base Atk +3; CMB +4; CMD 16 (20 vs. trip)
Feats Combat Reflexes, Iron Will
Skills Acrobatics +9, Bluff +6, Fly +9, Knowledge (Nature) +4,
    Perception +7, Stealth +9, Survival +2 (+6 scent tracking)
Languages Sylvan
Ecology
Environment foggy plains or forests
Organization solitary, pair, or pack (3–14)
Treasure incidental
Special Abilities
Vanish (Su): As a swift action, a mist dog can vanish for
1 round as if affected by invisibility. It can use this ability for
3 rounds per day.

Using Elephant in the Room, dex to hit is free.