"It does not seem very dangerous, but this infected girl is so fast that it can jump over the walls and kill any soldier with its sharp claws."
Harpies have the fastest move speed in the game and are definitely a force to be reckoned with. Because of its claws it has the ability to jump over walls which can easily catch the player off guard and infect lots of vital buildings. These spawn around the edges of the map and can also spawn in the last few waves.
Notice: Don't underestimate their wall jumping ability it can easily take out an army if not taken care of quickly.