She collects ceramic cats. Neither Dan, nor Amy enjoy being in her company. She has wrinkles and blue hair, and wears a lot of rouge and uses Day-Glo red lipstick. She is a very slow driver. As Grace's sister, she is, biologically, a Madrigal, but is not an active one, as she has not passed the "tests" as Amy and Dan have. She stays out of the Clue Hunt, and tries to get/keep other people uninvolved, thinking the Clues to be "a lot of rubbish". It was later revealed that the real reason she quit the hunt was because she was to afraid to take part.
She is shown to be angry at Amy and Dan because they both choose the clue over the million dollars, she was last seen storming off very angrily. She is known to have been pressured into adopting Dan and Amy because Grace wanted her to. Amy and Dan resent her to the most of their ability.
She quit going onto missions, because she couldn't handle them, unlike her sister, Grace.
She appears at the the beginning of the book at William McIntyre's funeral complaining and naming Cahills Amy and Dan never heard about. She leaves a few minutes before Amy and Dan were attacked. Her relationship with Amy and Dan becomes more healthy. It also seems that Amy is the one bossing Beatrice around now, not the other way around.
She appears in a conversation with Fiske, Amy and Dan Cahill. The conversation is about the outcast. Beatrice seems to agree with the outcast but is not fully on his side.
The Outcast invites her for a meal, and later, he takes out a syringe, and shanks her, and she dies.
- James Cahill - Father (deceased)
- Edith Cahill - Mother (deceased)
- Fiske Cahill - Brother
- Grace Cahill - Sister (deceased)
- Nathaniel Hartford - Brother-in-law
- Stephano Breglio - Ex-husband (deceased)
- Jean-Luc Benoit- Ex-Husband
- Sergei Baskov- Ex-Husband (deceased)
- Hope Cahill - Niece (deceased)
- Arthur Cahill - Nephew-in-law (deceased)
- Amy Cahill - Great-niece
- Dan Cahill - Great-nephew
- Alistair Oh - Distant Cousin (deceased)
- Bae Oh - Uncle (father's side) to Distant Cousin (in custody)
- According to Grace Cahill's Letter to Amy and Dan, Beatrice has no business in the Cahill family history.
- In the Cahill Web, it is mentioned that Beatrice has been named the greatest collector of ceramic cats in Massachusetts, and that she already had 3,024 of them.
- She apparently has a nasally voice that Fiske can imitate very well.
- All of her ex-husband's last names start with B, though this is probably coincidential.
- She possibly could have appeared in Trust No One because she is a suspect for V2, and the suspects usually appear in the books, however she did not appear in it.
- She has been on holiday with the British royal family.
- She did not speak for 30 years with her brother, Fiske.
- Even though she never liked Cahill heratige, in Nowhere to Run she appears to know alot of family members.
- In Nowhere to Run, she is less strict and it seems now that Amy is the one bossing her around.
- Beatrice is at least 87 years old in Maze of Bones, as Grace, her younger sister, is 87.