Angry Video Game Nerd I & II Deluxe

The video below shows all locations of the unlockable characters. I do not take credit for the video.

There are three hidden characters in the first Angry Video Game Nerd Adventures.

You will have to unlock them in a specific order, as each unlockable character requires another to access.

#1.) Guitar Guy (A.K.A Kyle Justin)
Guitar Guy can be unlocked in the stage 'Future Fuckballs 2010'. Play normally through the stage until you reach some blue and red disappearing/reappearing blocks. If you continue to the right, you will see more laser hazards. Continue to the right and avoid the lasers and you will find a couch with the skeletal remains of Guitar Guy.
Guitar Guy is not extremely useful. His attack is extremely weak, but can pass through objects. This will be necessary for the next character...

#2.) Mike (A.K.A Mike Matei, A.K.A Motherfuckin' Mike)
Mike is found in the stage 'Dungeons and Dickholes'. You will come to an area where you must open a green door by shooting the mechanism (as first seen in the tutorial). Just past this point, you will find another green door, but the mechanism is on the other side.
Just press or to switch to Guitar Guy and use his attack to fire through the wall and hit the switch. Continue on past a short obstacle course and you will find Mike.
Mike has a super powerful sword attack, but lacks range. He also has a very high jump which is useful for finding hidden items or shortcuts. Finally, he has the ability to see hidden blocks in select locations. Not only can he see them, but you will only be able to walk on them with Mike selected. If his jump wasn't awkward in navigating many of the game's obstacles, he would probably be the best go-to character. Finally, certain blocks can be destroyed using Mike's sword. This will lead to many secrets, including the final playable character.

#3.) Bullshit Man
Bullshit Man can be found in the stage 'Beat it and Eat it'. Play though the level until you reach a ladder that is right up against a wall. This particular wall is huge. Like, takes up half of your screen huge. Certainly looks suspicious. Hmmm.
Switch to Mike and you will notice right away that some blocks of the wall will turn a slightly different color. This indicates that his sword attack will break through the wall.
Keep breaking away at the wall until you reach the other side.
You know what's Bullshit? Bullshit Man's agility. He is slow as hell and has a double jump, but it only totals the height of the Nerd's regular jump. It does come in handy to find a couple of hidden areas that require the double jump to cleverly access areas that would be otherwise unreachable.
He also has what is probably the most powerful attack in the game. It doesn't have super long range, but can make quick work of enemies and some bosses.