What do the pets do? Do they give stats or any good in pvp Ranked battles?

No pets do not give any stats to your character. All pets do in the game is give you additional unique skills that only pets can learn.

These skills are:

• Guild summon - This summons your whole guild to your location (if you are in a guild)
• Party summon - This summons your party to your location (if you are in a party)
• Remote bank access - This opens up your bank remotely without having to go to a bank NPC
• Reduce EP consumption while dashing - This reduces and helps save/drains less ep while using the dash skill
• Remote shop - This lets you sell items to the NPC anywhere you are
• Self revive - Your pet revives you if you die and you re spawn where you died
• NPC teleport - this teleports you to an NPC of your choice on the map

Here is a tutorial from the old DBO form.

http://dbocom.com/forum/showthread.php?p=260316