Animal Super Squad

Slippy can be unlocked when you complete the Bonus Level Gritty Venture.

First, you will need to reach the 3rd World (Outer RYD IV). The 6th level of that world is called Gravity Room and you need to collect the collectable on that level to unlock Gritty Venture.

Here's a video to help you get through the hardest part of Gravity Room (at the end of the level with the Upside down arrow section). The trick is to boost at the right time and make sure that when you are upside down, to use and boost with . I think the hardest part will be at the treadmill but if you take your time and you are going in reverse (like in the video at 23 sec.) you will be able to reach the other side after a few tries. See my video below



You will then unlocked the bonus level Gritty Venture to unlock Slippy, the last character of the game. I made 3 separate videos of 3 hard/confusing parts of this level, feel free to watch them if you are stuck.
I want to point out that you can glitch out of the game at the end of the level when you reach the parts with the 2 stairs/buttons, much more easier than trying to complete this last part legit. Just boost into the sandfall and it should launch you into the sky, just boost to the right while being in the air. (Credit to Mikebut, see other solution)