It was early one morning in 1993, one year before the film's release, that costume designer Magali Guidasci picked up the phone to Luc Besson with the proposition of Léon: The Professional. “From the moment I first read the script, I realized it would be challenging,” Guidasci explains during a recent conversation in celebration of the film's 25th anniversary.

That challenge clearly paid off. A quarter-century on from its initial release, the film’s seismic impact on cinema and on style is still being felt. Back in ’93, however, as Guidasci was all too aware, working on the project required treading with an air of caution.

“We were introducing a very, very young character who would undergo a development and an arc that was so monumental, and all of a sudden the responsibility fell on my shoulders to do it right.”

Magali Guidasci with Natalie Portman on the set of 'Léon: The Professional' Gaumont Buena Vista International

Encapsulating the fragility of adolescence, the desire to love and the power of paternal instinct, the movie’s status among cult classics hinges on the complexity of its characters: 45-year-old Léon (Jean Reno) and 12-year-old Mathilda (Natalie Portman).

“I often start thinking about a character with the feet,” explains Guidasci, Besson’s right-hand woman for developing his youngest ever heroine, the avenging child assassin taken begrudgingly under the wing of professional “cleaner” Léon.

Beginning with a pair of scuffed leather boots and then cementing a few staple looks – “that jacket, which felt like the perfect middle ground between adult and kid” and the hair, which she describes as “new wave (’20s flapper icon) Louise Brookes” – Guidasci took to crafting Mathilda’s character slowly, before Portman had even been cast.

“I don’t know how many hundreds of girls Luc saw before he finally discovered Natalie,” she says, “but the day he showed me her tape, we were both like: ‘Woah. What just happened.’ It was the scene where she’s sitting at the table and she looks up at Léon and tells him she wants to kill. She had this piercing gaze. She was so engaging. So powerful.”

Gaumont Buena Vista International

Then just 11 (she turned 12 on set), Portman’s painfully honest portrayal of the fine space between childhood and adulthood, between girlhood and womanhood, struck a chord with everyone who saw it. Suddenly, Guidasci was faced with the immense responsibility of translating this into clothing.

“She’s a young girl who will soon enough be perceived as a woman,” Guidasci explains. “And my job was to protect that beauty of that, rather than fall into vulgarity.”

This delicate balance is what makes the whole film work, but it was also difficult ground to traverse, made all the more complicated by the desperate lack of cinematic references to draw from. “You had Lolita – that was absolutely not where we were going – and Taxi Driver. That was it. I had to find a way to support Luc’s story without crossing the line. And you have to involve everyone.”

Portman’s parents, particularly her mother, played an integral role in shaping the character Mathilda would become. But despite best efforts, Léon’s release subjected Portman to an onslaught of what she’s since called "sexual terrorism": fan mail in the form of rape fantasies and critic reviews about her “budding breasts.”

While the film was well received in Europe, it was undeniably controversial. Besson faced criticism for implicating such a young character at the centre of his narrative, with influential film critic Roger Ebert writing: "Always at the back of my mind was the troubled thought that there was something wrong about placing a 12-year-old character in the middle of this action ... In what is essentially an exercise – a slick urban thriller – it seems to exploit the youth of the girl without really dealing with it." It was later claimed that Besson’s own relationship with actress and model Maïwenn had been a catalyst for the plot. (The pair met when Besson was 29 and she just 12. They began officially dating when she turned 15, the legal age of consent in France).

Though the circumstances between these on and off-screen relationships were obviously different (Besson’s was romantic while Léon’s was fatherly), the reality of Besson’s personal life makes certain scenes, particularly one where Mathilda talks to Léon about the prospect of losing her virginity to him, unsettling to watch. In the US such scenes were deemed to be too provocative or inappropriate, and were deleted altogether.

Gaumont Buena Vista International

It’s something the rest of Guidasci's team were rightly cautious of and did their utmost to steer the narrative away from the predatory. Reno made a conscious decision to play Léon as mentally-slow and emotionally stunted to eliminate a sense of threat, and Guidasci crafted Mathilda’s aesthetic with the utmost sensitivity. While Mathilda’s public persona is darkly confident, the film’s more intimate moments depict the recently orphaned child wearing vest tops and elasticated shorts, laying bare her innocence and desperation.

“She was a little girl,” Guidasci laments. “And I felt very protective of that. I had to go back and think about when I was 12 in the south of France: What was I wearing? How was my attitude? Unfortunately, I think it could have been taken the same way.”

Watching the film 25 years later, Mathilda’s character is unarguably three-dimensional, ever-oscillating between moments of power and helplessness. And the most masterful aspect of Guidasci’s design is honing in on the space between the hard-edged hit woman and fragile young child, her impactful aesthetic serving as a reference point for an entire era.

“That’s really more than you can even ask as a costume designer,” she says.