Complex sentence: The hidden characters tend to be stronger than the other ones, and some are very easy to unlock while others are much harder, but eight of them are clones of the standard characters with just about the same moves and different graphics.
Split into simpler sentences: The hidden characters tend to be stronger than the other ones, and some are very easy to unlock while others are much harder. Eight of the hidden characters are clones of the standard characters with similar moves and different graphics.