It doesn't define its own characters; it uses the characters from the parent movie, but it has its own frame counter, display list, etc.
@ are we sure it doesn't define its own chars ?
The sprite implementation is divided into gnash::sprite_definition and gnash::sprite_instance.
The _definition holds the immutable data for a sprite (as read from an SWF stream), while the _instance contains the state for a specific run of if (frame being played, mouse state, timers, display list as updated by actions, ...)
1.5.4