Mickey is a non-canonical character, created for the 2022 miniseries Why Didn't They Ask Evans?, an adaptation of the Agatha Christie novel of the same name. He and his twin brother Sammy attend the carnival in Marchbolt, are given a red baloon and tasked to bring Bobby a beer, saying it had come from Mr Miller.
Spoiler warning: A spoiler is announced! The following section contains details about the plot of Why Didn't They Ask Evans?.
It is later revealed that the beer had been poisoned, but neither Mickey nor Sammy saw who gave it to them, because the man was wearing a mask.
Spoilers end here.