Shinobu is a girl who lives with Meme in an abandoned building. She loves donuts from "Mister Donuts."
She appears to be an eight year old girl, but she was originally a beautiful vampire who had lived for more than 500 years. At the end of events of Koyomi Vamp, after Koyomi realizes that having saved his master from a certain death, leaded to a future where she will kill humans (the "portable food" she calls them) to ensure her survival, Koyomi revolts against Kissshot in a bloody battle to death, where Kissshot chooses not to use her full powers (in order to have a proper battle with her servant). Later on the battle it is revealed that she pretended to turn Koyomi into a human again, by letting herself be killed by Koyomi. After the shocking revelation, he cannot bring himself to kill his master who was giving her life for his, and who cared for him at the point of risking her life to save his (from sunburn) at the beginning of Spring's Break . But to prevent herself from eating more humans, Koyomi asked for help to Oshino Meme, who in the end tells that as a happy ending won't be possible for everyone, then everyone will have an unhappy ending! While Koyomi himself cannot return to be a fully human, Kissshot Acerolaorion Heartunderblade was left in a weakened condition (where she is no longer able to eat humans, and needs her servant's blood to survive) with no powers or traces of her original personality and therefore have to abandon her "Kiss-shot Acerola-orion Heart-under-blade" (キスショット・アセロラオリオン・ハートアンダーブレード) name.
She was named Shinobu by Meme at the beginning of Hitagi Crab, suggesting her kanji name Shinobu (忍), having "heart" (心) under "blade" (刃), matches with her original personality.
In Nisemonogatari, she actually even talks, while in Bakemono, she didn't say even one word. She calls Koyomi waga aruji-sama, "My master". She also reveals to be really talkative.
Ever since "Tsubasa Cat" where she starts living in Araragi's shadow, she no longer needs to suck his blood on a regular basis in order to stay alive. She forms a physical connection with Araragi where they can feel each others emotions and senses. It is also later revealed that she can gain strength into an "evolved" form after sucking Araragi's blood more than the usual dosage, she does this in order to fight for Araragi when he confronts Yotsugi and Yozuru.
In Mayoi Jiangshi, in the alternate timeline, Shinobu went berserk when Koyomi was killed by Black Hanekawa so she swore to destroy the world and created the Jiangshi that has been swarming the town. It seems that she truly cares about Koyomi and when she saw Koyomi was alive in the alternate timeline, she started weeping blood. In the end, Araragi simply states that he will die together with Shinobu in the end.
Note: Aya Hirano was only credited on a drama CD and not for the anime. Please don't try to add her as the VA for Bakemonogatari.