This one is easier to find by accident you just gotta go and do the secret boss ???
without taking damage during the fight.
If you wish to know how to find this boss I can give a short guide in the following spoiler.
You must put 1 star in each perk (Peer, Drain and the character unique one) then move the windows to the spot they indicate. One you have moved the windows and see the names change into Peer! or Drain! you must use your character ability inside of Peer! then click on the popup. You will gain a new weapon and must fully charge it and use it to break the walls and escape the window. The boss should then start.
Once you have gotten the achievement the empty menu on the main menu will have a invisible button in it that will give you a Friend and close the game

This one has quite a long series of events to reach it but it is worth it as the character is cool.
First you must use the character you get from beating the secret boss. Enter timed mode and reach the secret boss again with that character while your ability is active and you have your own window then look around for a glowing .TXT file in the background of your small window. Once you get that you can die or win it doesn't really matter. Go back to the main menu and look at the new menu that should have appeared. Create a empty .TXT file on your desktop and feed that to the new window. It will output some the following text to that file that alludes to the next steps.

a file from you, inscribed what to do.

another for me, now one i can see.

if what you desire is to be known.

to my fourth eye it must be shown.

If you do not wish to figure out the riddle the answer is to take a .PNG file that you don't care about (You can get one from scrolling down in the credits page and opening that link and downloading one of the two backgrounds) and feed it to a level 4 Peer then set the new image as your background. You can then go to the main menu and rearrange the menus to be in the correct places (You can resize them) once it is all correct you will see Blip show up