Liam appears to despise platypuses, due to an experience from his childhood where he was separated from his mother. Since then, he hunts them for trophies. He also carries boomerangs as his primary weapons, and has given each one a female name and speaks to them. After first trapping Perry, he stated he would turn the platypus into a trophy. Doofenshmirtz knew this was going too far, only to be captured as well. Liam even threatened to make a "pharmacist" trophy out of him, leading the two foes to work together to defeat him. It should be noted that Liam is one of the few characters to stand a fair fight against Perry, as it was Doofenshmirtz who would ultimately stop him. He also seems to care about the Danville Botanical Gardens, as he calls it his natural habitat.
In the end, it is revealed that he is not from Australia, but from Pittsburgh, and his accent was fake.
Liam can be described as a heavy-built man with a thick brown mustache, black sleeveless shirt, and fingerless gloves. He also wears boots aided with spikes at the sides (referenced from the poisonous spurs that male platypuses have on their hind feet which are used for defense).
When he is first introduced, he is called "Liam O'Cracken". He is even referred to as such in the Disney Channel promo for the episode.