The woman called Mio was once considered to be only an assistant of a local blacksmith in the Himalayas. Bruce Wayne, while he was traveling the world to prepare himself for his future role, came across Mio while he was training with a famous, and considered mythical, martial-artist known as Shihan Matsuda. The two became close during Bruce's trips to the blacksmith and began to fall in love.
While Bruce Wayne was still training with Shihan Matsuda, their home was attacked by an unknown assailant. The assailant was able to fatally stab Matsuda while he was asleep, but in his cries of pain awoke Bruce. Bruce attacked the assailant, who proved to actually be Mio, but before he could get any information from her he was attacked by Matsuda's wife, who had paid Mio to kill her husband. The wife was then killed by Matsuda himself, with his last energy, and in his dying words told Bruce that this is what closeness will bring him. Mio, after the attack, disappeared before Bruce could talk to her.
Mio, after her assassination of Matsuda, returned to her master, who was in fact Ra's Al Ghul. Ra's scolded her for being reclus in her assassination, because her heart was with Bruce. Ra's then decided to train Mio in the way of the shadows, and leave her previous life behind her.
Mio, now going by the name of Penumbra, is unleashed upon the world as a master of the shadows, and goes to Gotham on an assassination mission to kill the Prime Minister of Bhutan. While she waits for the right moment though, she is spotted out of the crowd by Harper Row and her brother Cullen. Penumbra travels up Wayne Tower, while being trailed by Harper Row, to prepare for the assassination. Harper, though, is spotted by Penumbra and is about to be killed by the assassin when Batman suddenly smashes into the room, having been told by Alfred that Harper had trailed Penumbra into the tower. Batman and Penumbra fight, buying Harper time to escape the tower. While they fight, Batman begins to recognize Penumbra by her voice and asks her who she was before her training, which she does not reply. Below, Harper, who recognized how Penumbra uses shadows, shines a spotlight into the room, which takes away Penumbra's power as well as her costume, which was a construct of the shadows as well. Now in her civilian clothes, Batman instantly recognizes her as Mio. Batman tries to talk with her, seeing her as the girl he once knew in the himalayas. Penumbra, though, because of the fact that she does not know that Batman is Bruce, tries to resist him and falls from the open room and disappears.
Traveling back to Ra's Al Ghul's home, Penumbra, because of her failure, is sent to the cells to reflect on her mistakes. Upon exclaiming that Ra's needs her, Ra's says that he will never run out of assassins and she is taken away.
Penumbra is skilled assassin trained in a fighting style from Southeast Asia, giving her a good amount of hand to hand combat skill. She also has a great amount of weapons training involving blades, as seen by her ability to cut a bat-rope into pieces in mid-air
Penumbra also is trained in a mystic discipline from India that allows her to manipulate the shadows around her into constructs. These constructs can include clothes, such as her suit, weapons, such as knives, swords and even crossbows and multiple arms, which she uses to fight her opponents.