The main character would most likely be the dragon. But in addition to that, there could be a human or another magical being. For instance, it could be a young adventurer who stumbles upon the dragon in its dark state. This adventurer might be curious and kind - hearted enough to try and understand the dragon's situation and help it find the 'light'. There could also be an elder dragon or a powerful mage who has knowledge about the 'light' and plays a crucial role in guiding the main dragon towards it.
The dragon is surely one of the main characters. There might be a sidekick character, like a small magical creature that accompanies the dragon on its journey to find the light in the dark. And perhaps there's an antagonist, a dark force that tries to keep the dragon in the darkness and opposes the 'light' that the dragon is seeking.