Decima Mullins is the agency’s client in the main case of The Hallmarked Man. She is the owner of a restaurant, the Happy Carrot, located on Sloane Street (Chapter 2).

Strike first meets Decima at a house in Kent she has inherited from her great-aunt, called Delamore Lodge. Strike is unimpressed by the house which is cluttered, old-fashioned and dirty.  Decima is described as ‘a pale, dumpy woman whose long, straggly brown hair had grey roots and which looked like it hadn’t been cut in a long time’. She is wearing black track suit bottoms and a thick black woollen poncho (Chapter 2). 

Decima asks Strike to promise he will not reveal her location to anyone, and then goes on to say that she is the sister of Valentine Longcaster, Charlotte’s half-brother and friend. Sacha Legard, another of Charlotte’s half-brothers, is also involved in the matter she wants investigating. Decima explains that her surname, Mullins, is a ‘married name, from when I was in my twenties’ (Chapter 2).

Decima wants the agency to prove that the body of a man found in the vault of a silver shop five months previously is that of her missing friend, who she believes matches the description of the dead man. 

During the interview Strike is shocked to discover Decima is concealing a very small baby under her poncho. Decima pleads with Strike not to tell anyone about the baby, named Lion, who is three weeks old and the son of Rupert Fleetwood, the friend whom she believes is the body in the vault. 

Later, Albie Simpson-White, a friend of Rupert’s, tells Robin that Decima is ‘OK, the only decent one’ of the Longcasters.  He says that she treated her staff well, and is ‘a really good chef’ (Chapter 30).

Strike learns from Sacha Legard that Decima is, at thirty-eight, a lot older than Rupert, who is twenty-six (Chapter 36).

Strike and Robin meet with Decima in Quo Vadis to update her on the progress of the case. Strike notices that she had lost a lot of weight since they last met and ‘her large brown eyes were shadowed, but she’d brushed her hair and dyed its grey roots’. Decima remains convinced that Rupert is the body in the vault and wants them to rule out the other two men who are possibilities (Chapter 49).