While it's currently not possible to turn a character so it faces the back of the scene (180 degrees), it's possible to use the following workaround to obtain a similar result.
1) Open the Asset Library and select a character from the Characters tab.
2) Place the character in front of all assets in the scene and resize it accordingly.
3) Click on the Props tab in the Asset Library and type in "shapes".
4) Select the appropriate shape to cover the parts of the character (face, clothes details, etc.), following the lines of the character.
5) Match the shapes color to the colors in the character using the Eyedropper tool.
6) Hold the SHIFT key and click to select multiple assets, then right-click and select Group.
Note: For better results, we recommend using static poses or simple actions (where the character doesn't move much).