Jaimie Alexander and Peter Facinelli
04/16/2013 at 06:00 AM EDT
and girlfriend Jaimie Alexander
keep each other close – and their pets even closer.
The couple attended the Onyx & Breezy Foundation's Saving Tails event in Los Angeles at the private estate of James Costa on Saturday.
"Peter and Jamie walked in holding hands," an onlooker tells PEOPLE. "They were holding hands and kissed. They seemed so comfortable on the red carpet."
The source adds that Alexander – who just got a dog a month ago – had an ailing pup at home.
"Peter was worried about the dog and went home early for it," says the source.
Inside the event, guests – who included Emmy Rossum
and Frances Fisher
– sampled an all-vegan menu made up of meatless meatloaf and mushroom risotto and peanut butter and jelly ice cream.
The Onyx & Breezy Foundation is a non-profit organization that works to benefit the welfare of animals, which includes the funding of equipment for medical facilities, spay and neuter programs, disaster relief, puppy mill rescues, food, medicine and supplies, as well as the funding of shelters, rescues, foster groups and sanctuaries.