It's been quite a while since I played Okamiden, but you got enough wrong that it hurts.
First, this is subjective but I would say that it's a bit more than just fine, it's probably the best game of it's kind in the DS. While overall inferior to the original (inevitable, giving the inferior platform), it has one thing that it does much better than the original and is it's main gimmick of the brush (probably the only reason why they made this game for the DS, honestly)
There are six total partners, although the first one is Issun and he's truly useless. Chibiterasu CAN fight on his own, there are even different kinds of weapons, like swords and disks which play differently. The partners add to your damage significantly, but there are even short segments where you have no partner (when you lose a partner due to story reasons and before you get the next), for not mentioning that Issun seriously does nothing, is not until Kuni that you get your first real partner.