Ring | Attack on Titan | Annie Leonhart

Attack on Titan is a Japanese manga series written and illustrated by Hajime Isayama. It is set in a world where humanity is forced to live in cities surrounded by three enormous walls that protect them from gigantic man-eating humanoids referred to as Titans; the story follows Eren Yeager, who vows to exterminate the Titans after they bring about the destruction of his hometown and the death of his mother. (from Wikipedia)

Annie Leonhart (アニ・レオンハート) is a graduate of the 104th Training Corps and former member of the Military Police Brigade. Her exceptional skills with swords and unarmed combat earned her the 4th rank, but she is also noted to be a lone wolf who struggles to work with others. She possessed the ability to transform into a Titan known as the Female Titan.

During risky missions, it is implied that she always wears her silver ring, which has a small folding blade, on her right index finger; she uses the blade as her primary means of causing self-injury to transform into her Female Titan form at any time.