The Informant is an unarmed civilian bribed by the GLA to spy on enemy forces. They can be used to monitor the enemy early in the game where the enemy doesn't have stealth detecting units yet. They also have the ability to report back to the GLA to increase their sight range for a brief period.
The Informant reports back his current situation and finding to the GLA command, doubling his sight radius for 15 seconds. 120 seconds cooldown.