Kanna is the first incarnation of Naraku to created, though technically the 2nd to appear, since Kagura appeared first. She has the power to absorb attacks and souls into her mirror, and later displayed the ability to summon a glass guardian to copy attacks of her enemies.
In comparsion to her sadistic siblings, Kanna displays no strong emotions. She is for the most part, emotionless and stoic, though when she dies, she briefly displayed emotions. This is surprising even to Naraku, who initially thought that because she lacked a heart, she didn't have any emotions.
Kanna first appears when manipulating a village in order to absorb Kagome's soul. When this backfired, her role diminished to that of a scout and an informant to Naraku. In the Final Act, she finally makes her move and summons a glass guardian to battle Inuyasha. This led to a difficult fight with her, leading to her defeat.
However, Naraku decided to have her sacrfice her life in order to kill Inuyasha and his friends. Before her death, she imparts some knowledge on how to defeat Naraku to Kagome, telling her that the light will destroy him.