Satisfy the corresponding requirements to unlock additional modes and abilities, as outlined below.

Clear one campaign - 8 skill arrangements (up to 3 skills per arrangement) for on-the-fly usage in campaign mode
Clear two campaigns - Unlock Steel Beast map in Mercenaries mode.
Clear three campaigns - Mining the Depths map in Mercenaries mode.
Clear all 3 default campaigns - Ada's story campaign
Clear Ada's Campaign - Ada unlocked for Mercenaries Mode
Clear all 4 story campaigns - Infinite...