Tale of Diep:Companions

Companions are a game mechanic added as part of The Tale of Diep expansion. They manifest as the 'pets' of this game, following their master player in a conga-line-like formation should a player have more than one active. They are first unlocked by completing your first run of the Polygon Playground game mode, and you receive your first random Companion afterwards.

Overview
Companions are small Polygon-like creatures that follow players that found them. Companions are unlocked in various ways after being first introduced, but a player will always receive their first Companion after completing one round of the Polygon Playground game mode. There are four Companions that have a chance to be unlocked after your first run of Polygon Playground, and the next three successful runs will get the other three.

A max of three Companions can be used by a player at one time, but a person can collect all types of Companions available from the Companions section on the menu between games. It is found below the Tank Database and Achievements buttons. Every Companion has a different effect. There are currently twenty-seven Companions in the Tale of Diep, all unlocked in various ways. Companions follow relatively in a line behind their player.

Companions
Note: For Companions that attempt to kill things, the XP will go to the player in charge of them. Second Note: Even if tasks required for unlocking Companions were completed before having access to them, the unlock requirements must be done again.
 * Square Baby: A small yellow square with a light blue aura around it. Every few seconds, spits out a Yellow Square. Every square has a very small chance to be a Green Square. Unlocked by completing your first/second/third/fourth run of Polygon Playground.
 * Turret Boy: A small gray Twin minion slightly larger than that of a Factory. Will fire bullets that inflict Broken for 0.8 seconds at the closest enemy. Enemies must be on your screen for Turret Boy to shoot at them. Unlocked by completing your first/second/third/fourth run of Polygon Playground.
 * Bomb Devil: Small black circle with a sharp-toothed grin, red happy eyes, and two horns. Will occasionally drop a black and red Trap that explodes into a ring of sixteen bullets that travel quite a far distance. Unlocked by completing your first/second/third/fourth run of Polygon Playground.
 * Little Buddy: A mini Smasher that gets provoked when an enemy gets within 20 tiles of it. It will flash red, and then fling itself quickly out in the direction of the enemy, doing Body Damage on contact. If it misses, it will return like a boomerang after travelling twenty-five tiles. If it strikes something, it will bounce off and return immediately. Unlocked by completing your first/second/third/fourth run of Polygon Playground.
 * Thief: Small gray circle with a black bandanna-like bandit mask. Has a small Auto Turret with a lance. Will actively chase after the Polygon with the highest value within a 100 tile radius of it. Does 2x damage to Polygons, but will attack enemy tanks if they get too close to the player in charge of them. Polygon XP that the Thief gathers will go to you when he returns. Unlocked by getting the "Shiny!" achievement.
 * Glacier Square: Light blue shivering square that will occasionally fire icicle-like triangle bullets at the closest thing. Upon death by the Glacier Square, whatever that was killed will become a frozen statue, and can be pushed and will explode into a ring of sixteen more icicles upon hitting something. Unlocked by getting Paralyzed thirty times.
 * Little Brother: Small Basic Tank companion of the same color as the player. Mimics the attacks of the player in charge of it. Unlocked by playing every tank once.
 * Lil' Hive: A tiny little dark gray Overlord that summons tiny gray Crashers that will rotate around the player. Crashers will chase after provokers, but die easily. Unlocked by playing all Drone-using tanks.
 * Laser Eye: A light gray metallic circle with a glowing red light on one side. Fires a thin laser beam at the closest entity every 4 seconds, with infinite pierce, but a thirty tile range. Unlocked by beating Mad Science for the first time.
 * Baby Angel: A small white tank with two flapping angel wings and no barrels. Every few seconds, will fire a white beam into the player, healing them for 10% of their max health. Unlocked by being Number 1 in a team gamemode.
 * Invader: Black tank with spider legs hanging beneath it. Has four red eyes, the two ones on the ends being slightly raised. Fires four red bullets that inflict Slowed for 2 seconds. Unlocked by beating the Wallflower thrice.
 * Horseman Boy: A tank with mixed features of all four Final Sons. Is green like The Pestilent, has a helmet like The Warhand, uses a scythe as a weapon like The Harbinger, and occasionally summons Drones like The Siphon. Will charge at nearby enemies, and sometimes block a shot. Unlocked by beating all four Final Sons.
 * Multi-Tool: A miniature War Machine that follows the player. When provoked, will begin rotating around the player, utilizing mini versions of all the War Machine's various weapons. Unlocked by beating the War Machine and War Machine II.
 * Pentafiend: A blue Pentagon with an Auto Turret on every side. Shoots at the nearest enemy and releases a ring of sixteen bullets when shot at. Unlocked by besting the Polygon Mother.
 * Wounded Tank: A light gray tank with white bandages, red and dripping in locations. As it follows the player, leaves behind a trail of red tiles that damage those who travel through it. Cannot shoot. The red tiles vanish after four seconds. Unlocked by escaping death with less than 5% of maximum health.
 * Lil' Defender: Instead of following behind the player, Lil' Defender rotates around them. Slowly rotates while releasing Traps from the Trappers on its three sides. Will fire at the closest enemy with the single Auto Turret on top. Unlocked by beating all Polygon bosses.
 * Cambion: Small black tank with its left red eye covered by a white bandage that wraps around the rest of the tank at a slight upward angle. The other eye glows red. Every few seconds, will fire a black Missile (the bullets that shoot bullets) which inflicts Burning to those hit. Unlocked by beating Adversary and Second Witness.
 * Baby Midas: Little golden Smasher with a silver crown symbol on it. Acts like Little Buddy, but turns those it hits into golden statues with Paralyzed, Stunned, and Invincible for 10 seconds on top of taking damage. Golden statues will not recover health while frozen either. Unlocked by beating Boris the Ghost Butler.
 * Rage Minion: Red minion tank with an angry face on it. The lower health the player is on, the more damage, speed, and faster fire rate Rage Minion has. When the player dies, Rage Minion will chase after the killer at a rapid speed and fire rate until death. Companions can not normally be killed and vanish when the player dies, but Rage Minion is an exception. He gets 400 health points to seek vengeance with. Unlocked by dying twelve times in twelve minutes.
 * Revenant: Small black tank with a dark helmet and glowing purple eyes emerging from said helmet's shadow. Mummy wraps hang beneath it and the empty Auto Turret circles at its side. Upon death, the player has a 50% chance of being resurrected at half health three seconds later. The killer will not get points until those three seconds pass, and will only get the points if the player truly dies. Each time Revenant's effect occurs, it has a 5% less chance of happening until the player really does die. Unlocked by cheating death.
 * Apothecary: Small tank the same color as the player with a shield on the front and a barrel on the back. Will reflect bullets that hit the shield, and will occasionally give a random buff to the player for 5 seconds by blasting them with its barrel. Unlocked by using fifteen Totems
 * Lil' Nostra: A tinier silver version of Nostradamus that will slam its fist into the ground, creating golden shockwaves in all directions, that push away those hit and inflict Stunned for 6 seconds. Unlocked by beating Nostradamus.
 * Mr. Cult: A small black tank with the robes of a Cultist Dungeon Crawler. Acts just like the Cultist enemies, firing three fireballs in quick succession that explode in a 5x5 tile radius before warping to another place and doing it again. One of the few Companions that doesn't follow behind the player. Unlocked by besting the Tower of Gladii.
 * Guard Heart: Beating red heart-like entity that will become the source of taking damage for the player. The only way to hurt and kill a player is by shooting directly at their Guard Heart. Hitting their actual body does no damage. Great on some tanks, horrible on others. Unlocked by doing an entire match of a game with a win condition, without taking damage.
 * Baby Spirit: A small white spirit-like entity that can only take one hit before vanishing. The longer it survives, the more bonuses the player gets. Once it dies, all bonuses are lost and the Baby Spirit will not reappear until the player dies and comes back. Unlocked by becoming a Level 45 Spirit for the first time.
 * Lil' Siren: A small black tank with two white eyes, horns, and long black hair. Will fire out pink musical note bullets that attract entities toward you, luring them to their death as they draw closer to you, unable to attack for 4 seconds as well. Unlocked by beating Stormbringer, Wallflower, and The Heretic.
 * King Companion: All other Companions will follow King Companion. When the player is attacking, King Companion will stop moving, grouping all Companions together. Attacks by firing coins like Boris the Ghost Butler, that Paralyze for 0.9 seconds. Has an aura around him that boosts the damage of all other Companions by 1.5x. Unlocked by retrieving all other Companions.

Trivia

 * Mix of inspiration from Pets from Realm of the Mad God, and Familiars from Binding of Isaac.
 * The unlock requirements for Lil' Siren are a reference to the fact that it is similar to The Siren, a boss from Binding of Isaac: Antibirth, of which the three bosses that must be beaten to unlock it are also inspired by bosses from that mod.
 * A Cambion is the offspring of a human and an incubus/succubus in some religions.