Vanity pets in HKO will be a bit more useful. It seems that not only will they follow you around, but some will also be extra bag space. I can't tell you how much I wish my WoW pets would carry stuff for me. Sanrio hints that they will have other uses as well.
How will you get pets in Hello Kitty Online? Some monsters will drop "pet cards" which will allow you to have that monster as your pet. Also, there will be a way to acquire some Sanrio characters as your pets. This may require a lot of reputation or perhaps an extended quest chain or both, judging from the use of "eventually" by Sanrio.
And you can also have other players as pets! Kinda. But that's another story.
Which Sanrio character would you want most as a pet? Do you wish you could have a Ragnaros pet carry your bags around? I know I do.