When you create a character with Harmony, you have to connect all your nodes in the Composite node. The Composite node takes all the different images coming out of your drawing nodes and flattens them into one single image.
This is useful when it is time to apply an effect to your entire character or group your layers for a more organized network.
The regular Composite node will flatten all the pieces into one image preventing interaction with props or characters connected to a different Composite node. That is why you have to change the Composite node's type to a special one that will still output one single image but will not flatten the drawings together. This will allow you to nudge your drawings on the Z-axis so that parts of you characters can pass in front and behind other characters and objects. This Composite type is called Pass Through.