Sassette Smurf is only available on the island and she works there as Smurfette does in the main village. Sassette is not stationary, but walks around the island. She comes with her own mushroom house that needs to be bought for 30 Smurfberries. After that Sassette will live on the island. Just like Smurfette she gives extra XP to all Smurfs close to her when she has a red heart sign above hear head (which happens every couple of hours), but unlike Smurfette, who gives 15 XP per Smurf, Sassette gives 40 XP to every Smurf close to her.
But the area that she affects is rather small, so it is quite hard to get many Smurfs close to her at the same time. To collect the extra XP, just tap on her when there is the heart shaped sign above her head.