DracoFighter features 4 secret characters, which are hidden for multiple reasons. First, they're all non-canon. As original characters of the game's creator, they don't actually appear in DracoFighter's world or story, instead having their own stories outside of the game. Second, they don't share the game's philosophy. DracoFighter is meant to be a simple game to play, but the secret characters are generally more difficult and complex, despite sharing the same control scheme. Third, they have no intention of being finished. Their sprites and programming are functional, but weren't given the same level of polish as the main characters since they're largely meant to be experimental to explore the limits of DracoFighter's engine.
How to Unlock
All the secret characters are accessible from the character select screen, and don't need to be unlocked through anything like outside achievements. While the creator of the game prefers players at least attempt to figure out how to unlock them, for the sake of completion, highlight the text below to figure out how to choose the secret characters: Highlight/Hover over one of the 4 main characters, then press Grab, which is G on keyboard by default. Since mobile players don't have access to a keyboard, a hidden button on the very top left of the screen can be tapped instead. Hovering over a character on mobile can be done by holding a finger over a character, moving the finger off the button, THEN lifting the finger off the screen to avoid registering a tap. Each secret character corresponds to each of the highlighted main characters, and will be listed in the Characters section by the order they appear in the character select screen (Draco, Plexie, BiOS, Cyrus).