Diane Makepeace was the daughter of Keelan Makepeace, the perpetrator behind the Jigsaw Puzzle Killings.
Like her father, Diane has 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 runs across the right side of her head, giving her a very mysterious, alluring look. Her hair is a pale shade of blonde, which she ties up in the back as a ponytail while allowing her hair to grow out small bangs on both sides of her head.
Diane has inherited a devious, murderous streak from her deceased father. She was able to mastermind a murder in South America simply by manipulating her fiancé into murdering an archaeologist, while disguised as a local by the name of Mariana Etista. Separately, she was also capable of murder by herself. She also presented a somewhat morbidly playful side of herself, going so far as to re-create one of her father's murders through murdering.
On the surface, however, Diane presented herself as amicable - at least, to Lucy Baker. She had a calm demeanour that belied her quest for vengeance, seeking to end Alfendi's life whatever the cost because she had thought that Alfendi was her father's killer. She was later proven to be conclusively mistaken, though it was too late for her by then.
Diane was the daughter of Keelan Makepeace, the man responsible for the ruthless Jigsaw Puzzle Killings, and was personally taken to one of his killings, where a photo was taken as memorabilia. She was informed once when he was drunk that he can get away with the killings because he had connections with Scotland Yard.
Diane never knew for certain who one of his friends in the force was. However, after Keelan was killed at Forbodium Castle, she was convinced that the Inspector Alfendi Layton, the prime suspect, was the one he had connections with and he had betrayed her father.
- Main article: Mariana Etista
Three and a half years later, she disguised herself as Mariana Etista, a young woman fluent at English, and worked at a hotel ran by Micah Sasucasa. She soon became the bride of 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. The rest of the Agonni family had thought 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, who was on the Jigsaw Puzzle Killings investigative team, somehow became aware of Etista's motive, and thus had Alfendi investigate the Mansion.
After greeting the Mystery Room pair in the mansion's hidden basement, Diane escaped through the old, hidden subway that was adjacent to the mansion.
As an endgame, Diane threatened Alfendi to look for her at Forbodium Castle, warning him that she will bomb "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 - the millionaire's murder - ostensibly to prove that she knew of Alfendi's apparent "corruption". After that, she had her word with the professor, believing it to be her farewell to Alfendi as she prepared to shoot Alfendi 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 lithe 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 feeling that was not shared by the others.