03/28/2011 at 01:05 PM EDT
Looks like someone's having growing pains! Dog lover Emmy Rossum is still getting used to her adopted stray cat, Fiona G. Kitty
"She gave me a good clawing and biting on Christmas Day," Rossum told PEOPLEPets.com. "It's tough with any animal that you're integrating into a house."
The actress took in Fiona, pictured above with Rossum and her Shameless
co-star Ethan Cutkosky, off the streets of Chicago in December 2010. It seems her newest addition is having some difficulty adjusting to life indoors – and to having a sibling.
"She's used to being outside, so she really wants to be outside," Rossum said. "She's run away a couple of times from my house." Sharing a home with the star's pet Yorkie
has also been an adjustment for Fiona, though Cinnamon seems to be taking it in stride.
"I think she tries to play with him. She tries to punch him in the face and then run away and see what he'll do, but he just takes it. He's whipped," Rossum said. "They're getting along better [now], but the cat is growing and is now bigger than my dog, so she's definitely a tough cookie."
While Fiona may have a ways to go before she's fully housebroken, Rossum's made progress on her greatest goal – potty-training
"We're working on it," she said of teaching Fiona to use the toilet via the CitiKitty system
. "[The litter box] sits inside the toilet and they learn how to jump up and do their business there. Eventually, you just take that out and they are all acclimated, so that is the goal." Read more about celebrity pets on PEOPLEPets.com:'Big Bang Theory' Star's Cats Are Like 'Little Tigers'Video Flashback: Elizabeth Taylor Introduces Her New Puppy