The assistant leader is a role given to a Smurf with great leadership skills among his fellow Smurfs in various fanfiction stories.
Empath: The Luckiest Smurf stories
In the EMPATH: The Luckiest Smurf story series, the role allows that Smurf to oversee all activities in the Smurf Village in the same capacity as Papa Smurf and would give orders as well as relay any specific orders from Papa Smurf. The similar role of assistant counselor, which was given to Empath shortly after his final return from Psychelia, is more in the manner of providing specific counsel unto a Smurf who asks for it, helping to resolve situations that that Smurf may be having with himself or with other Smurfs.
Originally Hefty was assigned the role of the assistant leader around the same time after all of Papa Smurf's fellow Smurfs have died, leaving him as the sole parent of 97 young Smurfs. However, Hefty's behavior problems concerning how he treated various Smurfs that were under his charge -- particularly Grouchy and Empath -- have resulted in his role being taken away from him and given to Brainy, which most of the Smurfs did not like only because Brainy acted rather bossy. Eventually Empath was trained by Hefty and Brainy to replace them both as the assistant leader, with Hefty taking the lesser role of the supervisor. However, Empath can turn over the assistant leadership role to Hefty whenever it's necessary to do so.