WARNING: Spoilers!

This is a simple guide to help you get every character, if you're stuck on finding one. Also includes some alternative character names that are either things I like to refer to them as, or lore based names. 

Chapter 1:

Noob - Unlocked at the start of the game.

Lev - In Main Path 1, in plain sight.

Alt: Red

Keys - For defeating enemies in the Wicked Room.

Alt: Microwave

Cy - For defeating enemies in the Archon Room.

Mint Shard - For defeating enemies in the Drifter Room.

Alt: Mint

Dasher - For defeating enemies in the Hacker ROom.

Alt: DSH-R 89798

Rolo - Defeating enemies in the Joker Room, and defeating them as a boss.

Alt: R

Alex - In Main Path 2, towards the right side of the room. 

Wait - Defeating enemies in the Heretic Room, and defeating them as a boss.

Alt: Not Sleazel

Sleazel - For defeating enemies in the Muggle half of the Muggle/Thief room.

TD - For defeating enemies in the Thief half of the Muggle/Thief room.

Alt: TD the TV

Kin - For defeating enemies in the Patron/Keeper room.

Crimson - For defeating a few enemies in the Spectre room.

Alt: CDS

The Drone - For defeating enemies in the Spectre room. Required for progression, not a true party member.

Suggested name: Ark

Andrew/Andro - Flip a switch in the Thief/Muggle room, and then go to the Main Path 1, where a new room opens up. In that room, you can fight him, and unlock him as a party member.

Alt: Andromeda

Dirk - In Main Path 1, fight him to unlock him.

Alt: Borson Grea

Seven - In the room where you fight Andro, there's a hidden shop nearby. At the topmost part of the shop, Seven is there.

Alt: Exprendable #7

Chapter 2:

Jesus - Forced during story.

Chapter 4: 

Nero - After completing Reeve's challenge in Noob's room, go to Patron/Keeper room.

Alt: Hilm

Rank - In the shop where Seven is found, you can fight them after finding their hint above the Joker room. Once Chapter 4 starts, you can talk to them, and they'll leave the shop. Find them at the room above the Muggle/Thief room, where the Sticky Gloves are required to climb upwards. Talk to them there, and they'll join you.

Alt: Shawn

Overmind - Defeating enemies in the Overmind Room, and defeating them as a boss. Overmind room is found past Mandrake's room, east of the grind room.

Alt: Jim 1.0

Z - After completing the Overmind room, the Architect room opens up above the starting room. By talking to the Architect on the right, Seven will talk about some events during Chapter 2. After this, take the teleporter to Dark Space where Seven met up with Andro, and find the hidden path to the east. 

Alt: [Omitted]

Chapter 5: 

Reeve - Forced during the story.

End of guide. Good luck, I guess.