Support Cards:

A player can have up to 2 support cards active at once.

A support card does not, itself do battle, however they can force conditions upon either side.
For example, here is a few ideas for support cards that I have:

Blizzard:
Generates a cold Blizzard which increases all stats of the user's Wind based cards by 25%, in addition to this,
any Firestorm card which is currently enabled will be extingluished by the Blizzard.

Firestorm:
Generates a huge column of fire which surrounds the battlefield which increases all stats of the users fire based cards
by 25%. The column of fire will also burn away any Earthen Gift card which is currently in play.

Earthern Gift:
Summons a non-combat druid who will gift the users cards with Earthern Gift, which increases all stats of the users
earth-based cards by 25%, and also can cancel out a Blizzard or Tsunami card.

Tsunami:
Creates a powerful Tsunami, extingluishing a Firestorm and gives all Water-based card's stats a 25% boost.

Healing Ward:
Summons a non-combat druid who will heal your 3 most damaged cards' HP by up to 50%. Can only be used once per 3 turns.

Summoner:
Summons a non-combat warlock, who will create a portal for your units to enter and leave the battlefield.
The player can replace up to 2 cards which are currently in play, this card can only be used once in a battle.

Team Standard:
Places a Standard on the battlefield. Increases the morale of your army, and increases all of their stats by 10%.

White Flag:
Forces one of your cards to surrender, therefore joining the enemy team. Can be strategically used when you know the
weaknesses of that specific card. (Will only work when the enemy has a free card slot on their deck)

Death & Decay:
Opens the gates of oblivion and allows lots of evil spirit power to pass through. Increases all of the attributes of
dark-based cards by 25%, and overrides Tsunami.

Holy Light:
Openes the gates of heaven and allows good spirit power to pass through. Increases the attributes of light-based cards
by 25%, and overrides Death & Decay.