Talk to Alvina at Darkwood Garden. She's a big cat and is located on a windowsill before the bridge leading to the second castle ruins. Upon joining, she will give you a ring (Cat Covenant Ring), which enables you to summon her to fight off intruders entering the forest. NOTE: There is a bit of a glitch concerning this Covenant. It is recommended to quit the game and start the game back up after joining the Covenant, to prevent NPCs from attacking you. Also, if at any time you gain souls while in the forest from any of the NPCs dying (even if you don't kill them), then you will be kicked out of the Covenant. It is recommended you do everything in the forest before joining, then join and kill 3 invaders to max the Covenant ASAP.