Ape Escape: On The Loose Cheats

If you get up to Specter on Monkey Boxing and defeat him you will unlock him for Monkey Boxing. (I suggest doing this with Sir Lucas,he has good defense).
Then once you go to the character select screen he will be there.

In Specterland, after freeing Natalie and the Professor, head to the go
kart track. Jake will be there in a car. Step on the red spot in front
of the car. Jake will drive towards you. Double jump over the car and
hit the blue spot on the back of the car with your stun club. Do this
multiple times to defeat him.

When you go to save the game, go to the view monkeys section (the floppy disk with a monkey on it). To view a monkey, you are instructed to press X, but instead you can also use Circle. Highlight a monkey and press either X or Circle. Move to the last monkey you have (you must have over two pages), then move back to the first monkey. There will be an "L" at the bottom left hand corner of the screen, but you cannot move back with L. To exit this glitch, press R once and when you press L it will be gone.