I like the K-9 troop: Ralph, Kevin, Fido, Terrance, Spo (yes I count Spo to it. I know he isn't a K-9 but he hangs around with Fido a lot). Especielly Kevin and Ralph, they put a smile on my face.
Fox needs and deserves a special mention. He is, at least for me, the one you can very easily relate to and I think it's really interesting to see how he grows up with the time.
I also like Tarot. She is really funny in her own way, be it as a psychic or not.
Max is just a big charmer. How could you not like that big smile of his?
I also naturally like Peanut and Grape and if someone wants to know the reasons...well...read the comic and read it from the beginning
From the demigods I like Kitsune the most. He is a big troll, charming and made sure nothing bad would happen. I think he was towards Dragon and Pete like a big brother, who wanted to teach his younger siblings a valuable lesson
That said there isn't a Housepets character I somehow dislike.