Who's the White-Blue spirit, and all the demons fear it?

Traft!

Damn right.

...

He's a complicated geist, but no one understands him but his angel.

Plan A is to beat opponents into submission with an enchanted Geist of Saint Traft before they can boardwipe.

Plan B is to amass a large spirit army, with tokens or otherwise, possibly post boardwipe, possibly using Call to the Kindred .

Plan C is to use the draw and card advantage inherent to the spirits themselves and associated creatures to ride to victory.

Per suggestion, I have replaced Azorius Guildgate, Rupture Spire, Vivid Meadow and Vivid Creek with Ghost Quarter, Strip Mine, Darksteel Plate and Prowler's Helm.