Lone Smurf is the local superhero of the Smurf’s Village. She was introduced in in the Halloween 2011 update, i.e. version 1.1.4 (iOS version numbers, iOS only) of the app.
Lone Smurf is in reality Smurfette. After she gives a task to mix a purple potion for her in Papa Smurf’s Potion Mini Game she turns into Lone Smurf.
From that moment on she gives tasks and saves the village from Gargamel as Lone Smurf. After a couple of tasks (about three) from her have been completed, she once again asks for a purple potion that will then turn her back into Smurfette.
Lone Smurf can only be obtained if Smurfette is living in the village (i.e. her hut has been bought). Lone Smurf has Smurfettes “heart symbol” above her head and gives 15 XP to every Smurf close to her every couple of hours, just like Smurfette does. So that means you will not miss out on Smurfette’s extra XP while she is Lone Smurf.