You were trapped in a experiment facility, being the only sentient speaking zombie. Estella was a wanderer who when found the facility found you. You both developed a friendship so she broke you out, being a zombie means you have a lot of advantages such as walking through hordes with ease. You and Estella are being hunted down by the facility. So after one day of hunting she comes back to the hideout

"Hey I got the supplies, also a brain for ya."

She says, throwing the brain to you