With one character being inherited from /mlp/ and the other being a random anon's OC, at one point some people decided /pone/ needed a board-tan designed by the board for the board. Thus was born Red.
Red is a cream-coloured earth pony (later redesigned as a pegasus because people liked the idea of all three races being represented, but it's generally left to individual preference how anyone wants to portray her) with a wild unkempt red mane and hair. She either has no cutie mark, as an allegory for both her and the board's as of yet unfilled potential, or, more commonly, a "spoiler" symbol to symbolise the board's one feature /mlp/ doesn't have: it allows porn when it's spoilered. The one piece of clothing Red always wears is a black bandana featuring 8chan's infinity logo around her neck.
Compared to the other board-tans, Red is fairly dour. She's cynical and unpleasant to most people under normal circumstances, with a particular hatred towards authority figures (especially Royal Guards), but she holds no grudges and as long as there's enough rum to go around, she loves a good party and will freely share her booze with anyone. Red claims to be one quarter "Saddle Arabian" (this may be a fabrication on her part), but other than an expressed disgust towards all things related to pigs it's not a part of her character. Red also swears like a sailor, appropriate given that she is a pirate. Aboard her ship "The Crippled Midget", she sails the seas and lives life to the fullest, not bothering to care about tomorrow as long as she's got wind in the sails, money in the chest and a bottle in hoof.
Red has a near-identical colour scheme to Ruby. This is no coincidence: the anon who offered the first concept of Red understood the board's desire for a collectively made mascot, but sympathised with Ruby and her fans enough to throw it in as a reference. Because of that, it is now generally accepted that Red and Ruby are sisters when they appear together. A more obscure theory is that they're the same pony: set in a scenario where Ruby's casino heist (see above) failed, Ruby would have been tortured, had her horn snapped off, and been thrown in a river and left for dead, until the current dragged her to sea and she was scooped up by a passing pirate ship. The shock of the experience would have completely changed her outlook on life, and Punished "Red" Ruby took up a new name to reflect that. In this scenario, "Red" is always depicted as an "earth pony" (with her hair obscuring the stump of her broken horn) with no cutie mark (which she would have painted over to hide her identity).