Mason appeared in "Genie Meanie Minie Mo", and helped Timmy Turner win against Norm the Genie to make one more wish. Since he and Norm seem to know each other, and do not seem on the best of terms, its likely that he has had experience undoing Norm's handiwork in the past.
He has curly black hair, wears a suit, and speaks with a Brooklyn accent.
Aside from his lone appearance as Timmy Turner's lawyer against Norm the Genie, he was also mentioned by Wanda in "Channel Chasers" when she asked Timmy "Who do you know you are, Fairy Mason?" when he tried to go around his parents' punishment of him using a loophole.
Fairy Mason is a reference to the fictional lawyer, Perry Mason.
Fairy Masons Danish name, Klaus Riskær, might be a reference to Klaus Riskær Pedersen, a fraud sentenced Danish entrepreneur and businessman.