Warning: There WILL be unmarked spoilers. If you do not wish to be spoiled, please do not read this.
Shihouin Yoruichi is a very skilled dark-skinned woman who is capable of transforming into a black cat at will. She is intelligent, knowledgeable, and has been playing a role akin to a teacher and instructor for Kurosaki Ichigo and his friends.
Most of the time, Yoruichi is shown to be very serious, although she does enjoy teasing Ichigo by transforming to her naked human form without warning.
Before Yoruichi left Soul Society, she was the Commander of the Covert Ops (also known as Special Ops and Secret Mobile Unit/Corps). And she was also from one of the Four Noble Families, making her a highly respected woman back then. But ever since helping Urahara Kisuke escape from Soul Society, she has lost her noble status and had to leave Soul Society as well.
Shihouin Yoruichi’s Powers
Perhaps the most notable technique that Yoruichi possess is the ability to transform into a black cat. It allows her to disguise herself and move to places where her human form might’ve caused difficulties. Her cat form also changes her voice to one that is deep and sounding like a male’s.
The other technique that Yoruichi was most often seen used is the Shunpo (Flash Step). The technique allows her to teleport short distances with every step she takes, resulting in very fast movements. While many high-level Shinigami such as Kuchiki Byakuya can perform Shunpo as well, Yoruichi is the best at it. Her Shunpo is so good that she is given the title of Shunshin (Goddess of Flash).
Yoruichi is also a master of hakuda (hand-to-hand combat) and kidou (demon arts). And combining the two, she has mastered an extremely powerful technique named Shunko (Flash Release) which she is yet being able to control well. The technique allows the user to concentrate high amount of energy in their body and blast it at opponents. This is also the reason why Soi Fon’s (previously Yoruichi’s) military uniform has no back and sleeves, as they get shredded to bits when kidou pressure builds up on the user’s back and shoulders.