Most West Coast and Fremantle fans would rather walk over hot coals than 'barrack' for the other WA-based team.
But one man switches his allegiance from the Eagles to the Dockers every week, although he manages to avoids a barrage of abuse for doing so because nobody can see his face.
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One man, four mascots
Simon Lumbus switches is the mascot for the West Coast Eagles, Fremantle Dockers, Perth Wildcats and Perth Glory.
The reason for the swinging loyalty is because Simon Lumbus is the mascot for both clubs.
At West Coast home games the 43-year-old father of three is Rick "the Rock", but when the Dockers play at Domain Stadium he is Johnny "the Doc" Docker.
"I understand i's a bit of a conflict of interest but I've been doing it for a while now," Mr Lumbus told Radio 6PR on Thursday afternoon.
"I keep it very quiet. When someone says they are an Eagles supporter I say, 'funny that I'm an Eagles mascot'.
"If someone says they are a Dockers fan. I say 'I'm the Dockers mascot and they love it'.
"If I tell them I'm both, I'm hated by everyone," he said.
The Guildford Grammar teacher said he had a different approach for each mascot.
"The Dockers mascot is a tough, fit, strong guy while Rick is more of a rock star who's just incredibly cool," he said.
But when it comes to the Derby, Mr Lumbus isn't torn about which costume to don.
"I've been doing Rick for 19 years and about 10 years after that the Dockers asked me to be their mascot too," he said.Â
"So I straight away I rang West Coast and said look it might be a bit of conflict of interest what do you think?
"Without dropping a beat they said who are you going to be in the Derby?
"So I'm the Eagles mascot during the Derby."
And, if people have noticed a number of mascots busting out similar moves at other sporting games in Perth, it is probably because Mr Lumbus is also the Perth Wildcat's mascot Wilbur as well as Perth Glory's mascot George the Glory Gorilla.
Mr Lumbus doesn't know how much longer he can leap around Domain Stadium, but said he has no plans to hang up the suits just yet.
"I used to do gymnastics a while ago and I was in great nick and backflips were easy," he laughed
"Right now it's getting a bit harder - gravity is not my friend and the suit gets a bit tighter every year."
Originally published on The Age as The dark secret behind the Eagles and Dockers' mascots.