If you've been following Overwatch lore and development pretty closely you almost certainly already know who Brigitte is. And past that you're undoubtedly aware of her move from Overwatch's comic history and into the game itself. Today Blizzard released a video headed up by Overwatch's Art Lead Ben Zhang, to explain exactly how Brigitte came to be and revealed she will join the game on March 20, 2018!
First off, she was already a character in the Overwatch comics. This is why her long brown hair and freckled face have made the transition into the game. The team wanted to stay loyal to her origins. After that the first development of her character for the actual game was a mechanic/engineer look. Everyone truly enjoyed this as it took direct inspiration from her history as Torbjörn's daughter, and her engineering beginnings.
However, this wasn't meant to be, because the next character they wanted to add into the game was a Paladin (a tank healer). The original design was a character covered in silver and gold armor, with a shield depicting a sun in the center. This was altered when the Overwatch team decided that Brigitte would be this new hero. Her new (and final) look took into account her history, how she would play, and what her current influences are. Brigitte is Reinhardt's squire so of course she packs a metal punch with her armor. However, she's also still Torbjörn's daughter. So a bunch of leather/brown fabric details have been added as well.
Zhang concluded this behind-the-scenes video by saying that the final design had to convince everyone, “that's her!” Seems they reached a happy conclusion for everyone involved in the development of Brigitte.
Source: Overwatch Twitter & YouTube