Diane Makepeace was the daughter of Keelan Makepeace, the perpetrator behind the Jigsaw Puzzle Killings.
Like her father, Diane had pale white skin and maroon eyes. While not disguised, she wore a very intricate dark purple-black frilly one piece dress, with matching arm covers, glovets and a black hair-piece that ran across the right side of her head, giving her a mysterious, alluring look. Her hair was a pale shade of blonde, which she kept tied up at the back in a ponytail while allowing it to grow out small bangs on both sides of her head.
Diane exhibited a devious, murderous streak much like her father. She was able to mastermind a murder in South America simply by manipulating her fiancé into murdering an archaeologist, while passing herself off as a local by the name of Mariana Etista. Additionally, she could be highly independent and was capable of murder solo. She also presented a somewhat morbid playfulness, going so far as to re-create one of her father's murders through arranging a caricature crime scene around a victim of her own.
On the surface, however, Diane presented herself as amicable - at least, when around Lucy Baker. She had a calm demeanour that belied her quest for vengeance, seeking to end Alfendi Layton's life whatever the cost in retribution for her father's killing. She was later proven to be conclusively mistaken in her assumption of Alfendi's guilt, though by then she was already deceased.
Diane was the daughter of Keelan Makepeace, the man responsible for the ruthless Jigsaw Puzzle Killings, and was present at one of his killings, where the two took a photo as a memento. She once witnessed her inebriated father claim that he could get away with his killings because he had connections inside Scotland Yard.
Diane never knew for certain who his 'friends in the force' could be. However, after Keelan was killed at Forbodium Castle, she became convinced that Inspector Alfendi Layton, the prime suspect in the death, was his ally, and that he had betrayed her father by killing him.
- Main article: Mariana Etista
Three and a half years later, she disguised herself as Mariana Etista, a young woman fluent at English, and took up a position at a hotel run by Micah Sasucasa. She soon became the bride in an arranged marriage with Chico Careta, a young man who was the local guide of an archaeology group led by Archie O'Logie. When the team dug up a stone idol, she convinced Careta to steal the idol by killing O'Logie. After he did as she told, he gave his bride-to-be the stone idol.
Half a year later, when the investigation of the walking corpse case was being re-opened by the Mystery Room, she convinced Careta to visit the UK, knowing that Careta would be found as the killer. While Careta was being investigated, she fled the village along with the stone idol.
Later, Diane started to date Barbarossa Sassina, the right-hand man of the Agonni family. Rumors spread among the Agonni family that Diane was a ghost. Soon after, in an attempt to raise the attention of New Scotland Yard, she orchestrated the Merseyshore Docks murders, recreating the scene to look like the first four Jigsaw Puzzle Killings.
In a second attempt at raising attention, Diane went back into her guise as Etista and feigned an attempt to sell the stone idol. Knowing that New Scotland Yard was hot on her heels, Diane lured them to a mansion. Hilda Pertinax, an Interpol investigator who had been on the Jigsaw Puzzle Killings investigative team, somehow became aware of Etista's motive, and thus had Alfendi investigate the Mansion.
After greeting Al and his new partner Lucy Baker in the mansion's hidden basement, Diane escaped through the old, hidden subway tunnels adjacent to the mansion.
As an endgame, Diane sent invitations to Alfendi and Hilda to Forbodium Castle, threatening Al with the prospect of detonating a bomb at "a school, a hospital or Lucy's home" if he failed to show up. She then lured Alfendi into a trap and made him watch Lucy solve the case that he had not - "Pig" the millionaire's murder - to ostensibly prove that she knew of Alfendi's "corruption". After that, she had her final word with the detective, believing it to be her farewell to Alfendi as she prepared to shoot him in the head.However, Justin Lawson had managed to evade Diane's attention by using the East Tower rooftop to access the West Tower rooftop through the ivy that runs across the Central Tower. Sneaking in from the top of tower, Justin put a bullet through the back of Diane's head, instantly killing her.
- The pronunciation of her name forms the phrase "die and make peace".
- Despite her diminutive appearance, her age was stated to be 21 in Case 9.
- Despite being threatened with death by Diane, Lucy maintained that Diane was simply corrupted by her father's killings and death, a sentiment mocked by Alfendi Layton while his "Potty" state fronted.