One easy way to do it is:
Make a "Main" scene to contain the "CharacterSeletion" and "Level1"
Once you select your character, save the index information to a variable in the "Main"
Then, you queue_free() the "CharacterSeletion" scene and load the "Level1"
Now, you can use the "Main" to create an instance of your character and join it into the tree.
If you still want to separate the scene, you can search "autoload" for more information.
Hope this can help you.