The main character of a Nitrome game refers to the character the player controls, either directly or indirectly. For example, even though the player controls the fan, the fan is not a character, so the hot air balloon is the main character of Hot Air. There can also be two main characters, like Turner and Enemy 585.
Main characters may also not be on the good faction, they may be evil and the player controls them to fulfill their evil desires, like the parasite.
In some games, the main character may be a tool or vehicle which has to complete the level by usually fixing or destroying things, like the flyswatter in Pest Control. There are also other characters that may appear in games allied with the main character, like Lady Snow Fox allied with Takeshi.
Sometimes, but usually not in most games, there might be evil characters which may act like the antagonist of the game. Only in a few Nitrome game has the player switched to control other players, these games being Pixel Pop and Ice Breaker: The Gathering. In some games, the player may be able to customize the look of the main character, like in Skywire 2 and Hot Air 2.