Her name is Johanna and she has a very big shield that she hits people with. Way to remain in character, Johanna!
Blizzard hasn't been doing a very good job of keeping Johanna a secret -- details have been leaking out about her for weeks now. And by "details," I mean the fact that she's named Johanna, and that she's a Crusader -- a character class that was added to Diablo III in early 2014. Still no word on what her in-game abilities will be like in Heroes of the Storm, though.
UPDATE: Kotaku reader Return of Samus pointed out that while Blizzard hasn't said much about Johanna's in-game kit for Heroes yet, clever players already managed to datamine up some info about it. Heroes Nexus describes her as a melee warrior with abilities that center around dealing lots of damage, and temporarily stunning or blinding her enemies while doing so:
All Blizzard had to say about her in a new trailer released today was that it will follow up with another trailer at the beginning of next week when the game's just about to officially launch -- this new trailer being solely dedicated to Johanna.
My hands are readily perched over the Q,W,E, and R keys on my keyboard in anticipation.
Johanna's lineage makes well-suited for a tanky warrior position in Heroes of the Storm. That, and the fact that the game already has a divine warrior/knight in shining armour type support character in Uther from the Warcraft universe: