NPPlayers

Info
NPPlayers is a system that lets the player choose one to three companions that follow the player and assist them in battle. They are characters made by you, but with a special AI for automatic actions. They're able to be toggled on and off similar to the head bobbing. To add a NPPlayer to your party, right-click the character you want in the character select screen. If you want to remove them from your party, right-click them again. NPPlayers can also be a good way to get lots of extra inventory space when raiding for long periods and its possible to move items from one player to a NPPlayer's inventory and backwards. They can also be useful for battle if properly equipped.

Party Tab UI

 * The character you are controlling is the one with the asterisk (*) in front
 * Follow me: basic NPPlayer action which will make your party follow you; if they get too far away from you, they will teleport to your location
 * Guard here: option to make the NPPlayer selected stand in its then current position and attack enemies if they get close. You can also right click the option to set the position in which you want them to stay (a little gray arrow will appear)
 * Wander: makes the chosen character wander about
 * Control character/Lead the Way(NT6): you can choose between using your main chosen character and any of the NPPlayers in the middle of the game, just click "Control Character" button at the Party tab. This will change the main character and will allow you to do anything like adding status, using npcs, move items equip, unequip, etc. and always respecting each character's class
 * The name in yellow is the character chosen, and their inventory, skills and status will be shown

Glitch

 * Occasionally when a player spawns, you become the npplayer. You will lose anything not in your hotbar or armor slots as well as your levels.
 * Sometimes when you use a mount, a npplayer mounts and unmounts the mount all the time (insanely fast!).

Extreme FPS drops
Lately more info has been posted by the players on a matter regarding extreme fps drops and weird lighting on the game when using this feature, these lag spikes can occur when:
 * Using a hellevator
 * NPPlayers spawning next to you (may be casued by the spawning sparkling effects)
 * Changing biomes
 * Biome names changing may also cause lag spikes
 * Npplayers using lots of skills around the screen
 * Light levels changing drastically
 * NPPlayers summoning pets or minions

Some solutions

 * Use a lower screen resolution
 * Make NPPlayers guard at your home and call them with the Follow Me command when you reach the desired location
 * Make sure that the npplayers aren't wielding any summoning items, as this could cause them to reach the summoning limit which puts more stress on the game
 * Close any processes running in the background

In battle
NPPlayers are extremely useful in battle, prepare an area before calling bosses and test by it swarming it with enemies and with the npplayers (this should give you a rough idea of what it's going to be like, do however remember that each NPPlayer acts as a normal player, and multiplies a boss's health)

NPPlayers in tModLoader
The closest thing to NPPlayers in tModLoader is the TerraGuardians mod. It doesn't gives you the freedom to have your own characters as companions, but you do have pre-made companions with different abilities, visuals and status that you can have following you.

It's somewhat of an upgrade of NPPlayers, but some people may not like the characters in It, so It's a question of taste.

Raccoon Trivia
I really loved making this system, even more since I usually play the game alone. Not only I had something to play the game with, but also had to gear them, so I had more things to do in the game.

I've even made a mod on tModLoader to add companions to the game, they aren't recognized as player characters.