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. After the events at the end of Koyomi Vamp, she was left in a weakened condition 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.