Long answer: (Yes, it keeps going.)
P2 pets...It's best to get into the individual characteristics of P2 pets by using the Wetpaint guide. But the basics are:
P2 are obtained thru the item mall, or buying from players who have used the item mall. They do not drop in game.
P2 pets can help you attack, have useful skills, can be set up to automatically defend you and themselves, tho they can not b set up to attack mobs that are not already attacking you. They can also be set up to attack any mobs you are attacking, even if the mobs do not get the chance to hit you. (This is useful for a distance fighter.) They can be set up to automatically pick up drops. There are several spots for them to be told to do things in the AI, but if i remember correctly, anything after the 3rd spot is glitched and wont work properly. I think its self feeding is also glitched. (Feel free to jump in and correct me here.) Ask someone in game to help you get it set up the first time as it's a bit confusing.
A high lvl p2 pet can really speed things up for you and take care of some tedious tasks like farming some extra skill points, gathering affinity, lvling itself.
P2 pets gain exp based on the lvl of the mob you kill. The higher the lvl mob, the more exp the p2 pet gains when you kill it. It is not capped by your lvl or whether the mob is a higher or lower lvl than it, so you can kill any mob and it will get exp. Boss mobs give it 4x the exp it would usually get for a regular mob of that lvl.
You can buy armor and weapons for your p2 pet at the pet merchant. Make sure you get the right stuff for your class of p2. (itchi/scratchi vs animal vs demon) It doesnt really matter if you use a mag glass on the stuff, as long as your pet is wearing it he/she will get a big boost.
P2 pets go thru 1 or more evolutions to more powerful stages. It must be at least lvl 50 before you can evolve it at the pet trainer. The skills it can learn change at each evolution and it does not keep skills from lower lvl stages. That said, the base stats it starts with at each evolution can be improved by lvling it beyond lvl 50 before evolving it. (If you have the patience for it.) Some pets actually have better skills in their baby form, tho their attack and defense is sadly lacking.
Some p2 pets you can ride at certain evolutions, tho you still can not fight with your weapon from their backs.
****Maxie: this pet does not fight. It sits there and lvls and absorbs exp that it stores for you. It does not lessen the exp you get, just pulls extra out from the last dieing breath of each monster you kill. Every once in a while you can ask for that exp back and it obligingly regurgitates it for you. Maxie is the choice for high lvl players as the exp gain slows down considerably when you approach the lvl cap.