B.Orchid is currently free in Killer Instinct, likely for the next week. Before her, Orchid's brother Jago, Sabrewolf, and Thunder were all made free to play for a limited time.
Similar to free games like League of Legends, Killer Instinct rotates in a free character per week, giving players the chance to enjoy the game without making any monetary investments. Of course, the fighters are also available for purchase, at which point they become permanently unlocked for your use.
The fighters are priced individually at $4.99 USD, though you can also pay $19.99 for the whole roster. Recently, Double Helix released Spinal, a ghost pirate and yet another returning character.
Orchid is one of the quicker fighters available in Killer Instinct, as well as one of the few women currently available. She can summon a fire cat as part of her move set.