An Accessory is a vanity item you can equip onto a class. You can equip different ones over the various loadouts the classes have.
Contents
Accessories[]
Season 0[]

Captain Bracelet[]
Requires 10 of any Volunteer skins.

Spartan Mask[]
Requires 3 of any Veteran skins, and 1 of any Honored skin.

Christmas Hat[]
Requires 1 Volunteer Whirlwind, Dirk, Stalker, Agent, Hydra, Judge, Scorpion, Shinobi, Destructor, Terminator, Prybar, Mongoose, Killmark, and Ghost.
Season 1[]

Adventure Hat[]
Requires 8 of any Jungle skins.

Bunny Slippers[]
Requires 5 of any Tiger skins.

Sheep Hood[]
Requires 5 of any Tiger skins.

Horizon Chase Slippers[]
Requires 3 of any Sunrise skins.

Pirate Set[]
Requires 2 of any Overkill skins, and 1 Diamond Ore Buzzsaw, Maelstrom, Widower, Piledriver, Persuader, Machete, Hurricane, Constrictor, Oni, Serpent, Brute, Mammoth, Haunter, and Redeemer.
Season 2[]

Cat Backpack[]
Requires 7 of any Tiger skins, and 7 of any White Tiger skins.

Samurai Helmet[]
Requires 2 of any Seaquake skins, and 1 Urban Cobra, Spectre, Slayer, Gremlin, Curator, Demon, Devil, Shinobi, Hammer, Heavy Metal, Commando, Ranger, Warden, and Drone.

Frog Slippers[]
Requires 20 of any Jungle skins.

Paw Shoes[]
Requires 10 of any Urban skins, and 2 of any Rose skins.
Season 3[]

Viking Helmet[]
Requires 10 of any Desert Skins, and 2 of any Forge skins.

Cuirassier Helmet[]
Requires 20 of any Hearts skins, and 1 Blue Flame skin.

Magma Mask[]
Requires 8 of any Magma skins.

Burning Feet[]
Requires 2 of any Red Flame skins, and 1 Forge skin.
Core[]

Gentleman Set[]
Requires 10 of any Gridline skins, and 10 of any Circuit skins.

Greydeath Mask[]
Requires 2 of any Greydeath skins, and 1 Gridline Thumper, Tormentor, Executioner, Switchblade, Twister, Firebrand, Tornado, Blister, Sawblade, Rhino, Rampart, Blackhawk, Intruder, and Redemptor.
Missions[]
Halloween[]

Pumpkin Head[]
Requires 6 of any Pumpkin skins.
Unreleased Accessories[]
Automata Mask[]
Added to the game with the rest of the Season 2 accessories, this appears to be a mask similar in design to Tank Elite's mask, but having the color scheme of Automata: yellow-orange, white, and black. This has no accessory icon at present.