Normally, you'd expect Mario to fight Bowser in order to save her (it was the opposite in Super Princess Peach, though).
In this case, however, Mario has to fight her.
Due to her being the ruler of the Mushroom Kingdom, it is assumed that she was the one who crowned Mario as king. After he ruined her kingdom, she too decided to exact revenge on him.
It is currently expected that she will be fought in her castle in the Mushroom Kingdom.
Although her battle isn't finished yet, a non-functional version of it can be found in later demos, starting with the Overworld one.
It seems to have two phases: One where she uses her Super Princess Peach graphics, and another where she undergoes a transformation, her sprites being an edit of the Goddess (known as Sophia in Tetsuya Nomura's concept artwork for the Warring Triad) from Final Fantasy VI. Her arena uses graphics from Stage 7 in Castlevania: Dracula X.
Her attack pattern is unknown thus far.
Poochy's graphics from Yoshi's Island can also be found next to her SPP sprites, so maybe he'll give Peach or Mario some assistance in battle.
Princess Peach has long blonde hair, blue eyes. For makeup, she uses red lipstick. For clothes, she is commonly seen wearing sapphire earrings, a pink evening gown with a large sapphire brooch on it, white evening gloves, red high heels, and her crown.
In terms of build, she is rather slender, being taller than the two Mario brothers.
Peach has been shown to be a caring, compassionate ruler, running the Kingdom as effectively as possible. She does get kidnapped by Bowser quite often, however. Even so, she is not afraid to fight against Bowser or any other menace that comes to the kingdom, if it is necessary.
In the Mario canon, she never seems to hold any grudges, not even against Bowser. Thus, her portrayal in Brutal Mario is quite out of character.
In terms of hobbies, she seems to enjoy cooking, usually baking cakes after the end of an adventure.