The Nerd is your default character and can collect the Superscope as a powerup to improve his power. If you shift characters with the Superscope on, it'll disappear. The Nerd can direct his shots and will shoot as fast as you can mash the button so he'll cause a lot of damage if the enemy is in target.

Guitarguy is found at the third stage, ♥♥♥♥♥♥♥♥♥ 2010.
When you first find the appearing/disappearing blocks there are four vertical lasers to the right. Go Right to find the Guitarguy... behind his new couch... Turned into a skeleton. I wonder what did that to him.
He can shoot through walls and jumps a block height less, but moves faster.

Mike is at Dungeons & ♥♥♥♥♥♥♥♥♥.
You need Guitarguy to shoot a crystal ball switch behind a wall to reach the place where he is hidden.
The place isn't missable, you'll see this switch in every run, but without the shot you can't get in.
He has a lightsaber that causes a lot of damage in short range against some enemies and his jump is slightly higher than the Nerd's, in fact, it looks like Luigi's jump in Mario 2. One thing to note is that he cannot move while attacking, he will still fall or ascend from a jump though.
His special ability is Secret Vision, which will reveal breakable blocks or invisible platforms near him. These platforms cannot be interacted with by anyone else.

♥♥♥♥♥♥♥♥ Man aka B.S. Man is at Beat it And Eat it.
When you get to the end of the first corridor there's a massive wall to the right, Mike can see and destroy this wall.
Bullshitman has the double jump ability, which permits some directional control the others do not have. He moves slower and jumps less than every other character, but his attack where he throws ♥♥♥♥♥♥♥♥ causes a lot of damage, it arcs like the ♥♥♥♥♥♥ powerup but goes farther and has a larger hitbox.

The scope of this guide is to assemble the whole playable roster as fast as possible, by the time I got the entire walkthrough ready Swift Hobo made a guide with all cameo locations with screenshots.

http://steamproxy.net/sharedfiles/filedetails/?id=180185467#

My other guide is a walkthrough that covers all the extras and achievements within the game and gives some strategy for the bosses on Normal and harder levels. We also have the cheat list that's already known.

http://steamproxy.net/sharedfiles/filedetails/?id=180407248

You can check these out for more information.