Jak is a weirdo from another universe as Ratchet and Clank have nothing to do with him, except that he appears a bit.

He is friends with Daxter, not that anyone cares.

He is a secret character in Ratchet: Madlocked, but you only find him if another person can actually be bothered to do cooperative player with you. He is a skin; just like Ratchet is a racer in a Jak and Daxter game, but the difference is that Ratchet definitely deserves better.

Ratchet & Clank wiki
For those without comedic tastes, the so-called experts at the Ratchet & Clank wiki have an article about Jak.