She was by Princess Charlene's side during Monaco's Sainte Devote festivities last month and has long been considered a pillar of strength for Prince Albert II while his wife was unable to attend royal events due to her ill health.

And so it will come as no surprise that Melanie-Antoinette Costello de Massy is considered a source of  'fundamental support' to the Royal Family of Monaco, according to Vanity Fair Spain. 

But the 38-year-old, who is the daughter of Prince Albert's cousin Elizabeth, is said to have an incredulous background, filled with ancestral quarrels and tales of alleged betrayal.

Melanie-Antoinette is understood to have been by Charlene's side during her comeback into the spotlight, after a period of illness and treatment for 'exhaustion'.

In January, Charlene, 45, smiled for the cameras as she stepped out of her car and was shown into a Mass service, a key part of the festivities which take place every year in Monaco, appearing relaxed with Melanie-Antoinette's company. 

The first cousin of Prince Albert II once removed was also pictured at his own side during engagements while his wife was away, Tatler reports, for events going as far back as the 2010s.

It would seem that she has taken over her mother Elizabeth's role as the royal core's confidante. 

The godmother of Princess Stephanie of Monaco - Prince Rainier III's youngest child - who was also bridesmaid at his wedding - has always appeared to be an anchor for the main royals.

However, despite the seeming ease and closeness with which Melanie-Antoinette slots into the royal circle, the family tree is filled with a history much more complicated.

It is alleged that her grandmother Antoinette de Monaco, Baroness de Massy - who was the older sister of Rainier - once hatched a plan to remove him from the throne through vicious rumours that he couldn't provide an heir. 

Here FEMAIL delves into Melanie-Antoinette's fascinating family history and how she hasn't stopped her from rising through the ranks.

PRINCE RAINIER III AS HEIR PRESUMPTIVE 

The family feud begins as far back as the 1890s, when Princess Charlotte, Duchess of Valentinois, was born.

Charlotte herself was heiress presumptive, having been the illegitimate child of Louis II and cabaret singer Marie Juliette Louvet.

She would go on to have two children - Antoinette and Rainier. 

Despite Antoinette being the eldest, Charlotte gave up her right to the throne for her only son. 

Elizabeth - Antoinette's first child with tennis star Alejandro Athenase Noghès - was Charlotte's grandchild when she was born out of wedlock in 1947.

Elizabeth was legitimised in the line of succession in 1951 when her parents got married.

Although the couple divorced after three years, they still went on to have two other children - Baron Christian Louis and Baroness Christine Alix. 

In 1961, Antoinette found love with president of the municipality's parliament - the Conseil National - Dr Jean-Charles Rey.

According to Vanity Fair, the couple hatched a plan to remove Antoinette's brother from the throne by planting rumours that his girlfriend at the time - Gisèle Pascal - couldn't have kids. 

They were understood to think this would encourage Monegasque society to choose her son Christian Louis - then only a child - as the expecting heir. 

In the meantime, this would allow Antoinette to serve as regent.

However, while the outlet reports that the gossip may have in part driven Rainier and Gisele apart, the plan didn't succeed, as he later met Hollywood star Grace Kelly. 

Antoinette's third and final husband was John Brian Gilpin. She married the British dancer in 1983, only for him to pass away suddenly, six weeks after the wedding.

The prince's sister passed away in 2011, aged 90 years old.

Despite her mother's reported attempts to 'steal' the throne, Antoinette's daughter Elisabeth-Anne De Massy seems to have worked hard to heal familial relations.

Not only was she a bridesmaid at Ranier's wedding to Grace Kelly but she was also asked to be a godmother to their daughter Princess Stephanie.

As such, she was allocated the same honourary positions her mother had once held and has also represented Prince Albert at royal engagements over the years.

This has now been passed down to her daughter - who has been dubbed a 'fundamental support' to the Royal Family of Monaco.

ONGOING STRUGGLES FOR CONTROL OF THE THRONE 

In 2021, a documentary also shed more of a light on other threats facing Rainier's place on the throne as Marilyn Monroe was said to nearly have been courted by the prince in order to save his principality.

Curiosity Stream's Royals: Keeping the Crown claimed the blonde bombshell was considered by royal aides as a possible wife for the ruler until concerns were raised about her unsuitable 'sex-bomb image'.

Instead Rainier went on to wed Grace Kelly, the darling of Hollywood, less than a year after first meeting on April 18, 1956. 

The two enjoyed a successful marriage and had three children together - including the current monarch Prince Albert II - before the actress-turned-princess sadly died in a car crash in September 1982 at the age of 52.

According to People, Rainier was in danger of losing control of his throne after World War II, so aides suggested he break tradition and marry an A-lister instead of a royal to help improve tourism in the principality.

A longstanding agreement with France meant the prince's crown was going to be lost if Rainier couldn't revive his county's primary industry of tourism.

The monarch was also an unmarried man without an heir to inherit his throne, so at 32, that was 'uppermost in his mind', according to University of St. Andrews historian Dr. Chandrika Kaul, who appeared on the documentary.

Admirers recognise her as the embodiment of royalty in any sense – a critically-acclaimed actress and the elegant consort of Monaco, as well as a loyal friend, loving mother, and stunning style icon. 

Ms Kelly was born to a wealthy family in Philadelphia in 1929, and went on to quickly climb the ranks of Hollywood, starring in Alfred Hitchcock’s Rear Window and High Society, a musical.

She won the Oscar for Best Actress for her work in the 1954 film The Country Girl, beating out fellow star Judy Garland. She also gained another nod for her supporting role in 1953’s Mogambo. 

At the 1955 Cannes Film Festival, Ms Kelly met Monaco’s Prince Ranier III. After exchanging letters in a hushed romance, he asked her to marry him days after Christmas that year.

During what reporters called 'the wedding of the century,’ then 26-year-old Grace Patricia Kelly became Her Serene Highness Princess Grace of Monaco in an intricate lace dress designed by MGM costume designer Helen Rose. 

In the years following their wedding she admitted that it was difficult separating Grace Kelly the actress and Princess Grace, the wife of a head of state.

She had three children with the prince – Albert II, Stephanie, and Caroline. 

Princess Grace died on September 14, 1982 as she was driving along a hairpin bend above the Mediterranean principality with her daughter, Princess Stephanie.

It was the same hairpin curve that she zoomed around in the 1955 film To Catch A Thief. In the film, Ms Kelly gushed: ‘Have you ever seen any place in the world more wonderful?’