None of Jak's work is free except the base game, it's a standard free-to-play model: download the basic game and 3 characters for free, and then show your appreciation and get more awesome content by buying it!
User-made content like custom outfits are all free in the User Outfits section of the store, and user-customised character models are free to download, but unless they are based off the basic models and hairstyles you will need to buy the original characters and hairstyles for custom versions to work. Which specific characters you need to buy are detailed on the right of the page of custom character you want.
PPP is a cool user-made addon that makes custom characters automatically from the ones you own. Either set it up to only use the default characters (Alec, Robin and Layla) or buy more characters!