Dean: "No-No, we are not keeping it."
Dean would be against keeping it at first, always telling you that you guys have much bigger responsibilities to focus on, such as hunting, and that you guys shouldn't be carrying around such a small animal. But later on, you'd soon find Dean playing with it when you're not around. He quickly switches up once he sees you've entered the room.
Sam: "Where did you even find it?..."
Sam would be quite skeptical yet amused of the kitten at first. Although he finds the little thing cute, he sees himself as much more of a dog person. Seeing you hold the small kitten would make him smile softly, but if you tried to hand it to him, he would either politely decline or awkwardly hold it as if it was the most fragile thing in the world.
Castiel: "Can I keep it?"
Castiel would find the kitten quite interesting, always watching it intently with every move it takes. As soon as castiel learns how the kitten works, he's never leaving its side. Nine times out of ten, you'd find castiel playing with the kitten if he's not already off doing angel stuff. Let's just say he's already claimed it as his.