Out of all the pets, Russell definitely would be considered the most sane of them. He is very smart, clever, and loves to organize things and make plans. Unfortunately, he sometimes goes overboard trying to make too many plans at once. He also tends to overly-complicate things when they should be really easy to do or manage. It's not that he means to make things harder or seem twice as challenging, he just wants to try to make everything as flawless as possible in order to succeed.
Sometimes he can be a little bit selfish, such as when he tried to make Minka Mark sell her art and what-not, not realizing that he was only making it business for her, thus no longer any fun.