Angelina is the widow of the late Baron Barnett, the queen of high society (which has nicknamed her "Madam Red" due to the color of her hair and clothes), and Ciel's aunt. When she's not invading Ciel's home in London unannounced or flirting with her many admirers, she actually makes a living as a rare, female doctor. She appears to serve mostly female and child patients. Though childless herself, she's told Ciel in the past that she considers him her own son, and appears to have been keeping watch over him since he reappeared after the fire. She has her own butler, Grell, and is constantly reprimanding him to be more like Sebastian.
Both Angelina and her older sister, Rachel, were in love with Ciel's father, the previous Earl Phantomhive. The Earl took notice of the bookish and shy Angelina, complimenting her red hair, which she hated. Angelina promptly fell in love with him, and began dressing more stylishly and acting less shy in the hopes that he would propose. However, the Earl married Rachel instead. Though Angelina never really stopped loving the Earl, she tried not to begrudge her sister her happiness, and seemed to truly enjoy the time she spent with the happy couple and their son, Ciel. She even got engaged to Baron Barnett, who loved her despite knowing of her remaining affections for Earl Phantomhive. In time, she became pregnant.
But tragedy struck. A runaway carriage trampled both Angelina and her husband. He died instantly. She survived, but her unborn child was killed, and the internal damage prevented her from ever being able to conceive again. Rachel came to take care of her in the hospital, and invited her to Ciel's tenth birthday when she was released. Angelina arrived at the Phantomhive Manor just in time to see it burn to the ground.
Soon afterwards, one of her patients came to her for an abortion, claiming that children were worthless. Something in Angelina snapped, and she followed the woman out onto the streets that night and murdered her. At that time she met Grell, who decided that she would be the perfect human to assist in his killing spree. He posed as her butler, and together the two murdered women as Jack the Ripper. Eventually, Ciel discovers the truth, and confronts the two of them with Sebastian. She nearly kills Ciel, but is stopped when she sees her sister's face in his. Grell urges her to get on with it, but she responds that she can't kill Ciel, and as a result, Grell kills her, saying she's outlived her usefulness and the is not "fit to wear red".
Madam Red is loud, outspoken, and flamboyant. She loves to tease people, alternately cooing over how lovely Ciel looks in a dress and shamelessly groping Sebastian's ass. At parties, she seems to bask in the attention her admirers shower her with. For an upper-class lady, she can be surprisingly dirty-mouthed (and minded!), leading to the funny story she tells the Undertaker having to be censored out. She also has the tendency to rag on Grell whenever he performs below her standards, which is pretty much always. Despite all this, though, she truly cares for Ciel and tries to mother him in the only ways she knows how. As a doctor, too, she has an above-average education, and uses this to her benefit in coming up with her various schemes.
Her grief and rage over losing her husband, child, sister, and the man she loved all in one fell sweep has left her slightly unhinged, though she hides it well. She takes out this anger on women who, in her opinion, don't appreciate their ability to bear children. All the same, she seems somewhat shaken by her actions, propelled onwards mainly through Grell's urgings.
In her funeral, Ciel calls her "Aunt Ann".