There is a coin counter on any creature you control except Athreos.
You control at least one additional nontoken creature if Reluctant Role Model is the only creature you control with a coin counter on it.
Steps
Activate Altar of Dementia by sacrificing the creature with a coin counter on it.
When the creature dies, Athreos's second ability and Reluctant Role Model's second ability trigger.
Resolve the Athreos trigger, returning the creature from your graveyard to the battlefield.
Resolve the Reluctant Role Model trigger, putting a coin counter on it if it wasn't the creature that was sacrificed, or another nontoken creature you control otherwise.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to the sacrificed creature's power.