In her personal armoury, Joanne Calderwood has a lightning hook, a windmill kick and a vicious left elbow that finished a fight last year, leaving her opponent reeling after a crack to the side of the head. She is the undisputed, undefeated badass babe (sorry, feisty young lady) in a sport once decried as cage fighting but now more properly recognised and accredited as mixed martial arts, or MMA.

It is fought in an octagon – less of a cage, more of a padded playpen – and far from being a savage free-for-all, MMA combines the disciplines of boxing, wrestling, Brazilian ju-jitsu, judo, karate, Muay Thai boxing and taekwondo.

JoJo, as she is known in the grappling world, is Scotland’s greatest hope for the sport, with an unbroken string of eight wins so far. At 5ft 6in and 115lbs, she is the perfect poster girl for the new strawweight class, and her relentlessness in attack has taken the pundits – and often her opponents – by surprise.

In person, the tiger is a mouse, so quietly spoken that we have to change seats in a Canary Wharf hotel lobby to escape the booming voices of men she could easily reduce to whimpering wrecks. And she is so serene with her oval face and huge pale eyes that when she confides that she transforms into a quite different person in the cage, you’ve already guessed as much.

In her sweats and white sneakers with sparkly toes and white ribbon laces, she looks harmless enough and yet somewhere inside her that violence is filing its nails and waiting for its next hot date. “My aggression just switches on and off,” she says. “I always say to myself, ‘This person is going to hurt you and try and take the win away’… I am most myself when I am fighting. Outside the cage, it’s not really me.” She even hates the obligatory swagger – in her case, just a walk – from the dressing room, when all eyes are on her, with only her theme song, Kanye West’s “Amazing” (“I’m a monster, I’m a killer”) for company.

Working out at the Griphouse gym in Glasgow

Underneath her woolly hat, Calderwood’s long, magenta-dyed hair is half shaved, half hanging loose, and braided for fights. The muscled top half of her right arm also looks like it means business, a canvas for a sprawling black tattoo of fighting ninjas and eagles spread across the undulating biceps “to show the dark side of myself”, but it’s also a clue to her sentimentality. Nestled there are three hearts for her mother and two siblings; and pieces of a puzzle, the other half of which is etched on her partner’s arm.

The myth of the fighter as troubled child or mean-streets scrapper doesn’t really apply here: Calderwood, 28, is from a close-knit family brought up in suburban respectability in Kilmarnock, 30 minutes from Glasgow. Her parents worked in the NHS, her older sister is now a maths teacher. As a child there was nothing unusual about her, except maybe an exacting self-discipline that saw her rising early for swimming training every morning before school, never missing, never moaning. When she attended a Thai boxing class aged 13, to keep her younger brother company, she was smitten.

“People often ask me if I was bullied but I wasn’t,” she says. “For me, the worst thing was turning up at school and finding nothing that I enjoyed. I wasn’t a difficult or aggressive child but I didn’t know how frustrated I was until I started hitting pads and feeling much better, calmer, happier. I didn’t have to prove to anyone that I was good at Thai boxing, it was just natural for me.”

Having left school with one Higher (the Scottish AS-level), she planned to qualify as a nurse and worked as an assistant in a care home, a job she loved, training at a local gym after her 12-hour shifts, exhausted but determined. “The old people were so funny… It was eight years ago and I still think about them every day.” Her big break was being offered a part-time position at Glasgow’s leading MMA gym, The Griphouse, now the place that feels most like home. She still works there, Tuesday to Friday, training every day with the guys – she jokes with a mischievous giggle that she gets them to wear dresses and act bitchy in order to better resemble her opponents.

Training with a sparring partner

Sparring with men forces her to fight defensively, so she relishes the chance to be on the attack in her matches. But, until the sport’s premier organisation the Ultimate Fighting Championship (UFC) – which signed her last year from Invicta, the female MMA promotions organisation – created the lightest strawweight class, Calderwood’s lack of poundage made it hard for her to secure fights. The first strawweight women’s champion’s belt will be won in the next series of the UFC reality TV show The Ultimate Fighter, a sort of slap-happy Big Brother in which Calderwood will be one of 16 women living and fighting in a Las Vegas location until a winner emerges to claim the title and a six-figure contract with UFC.

Calderwood enters the house next month for a six-week period, though the show isn’t screened until September. She could fight three times, before a final fought in front of a live audience. And the rest of the time? “I’ll be hiding. I’m quiet and I love sleeping. I don’t like drama, I just like fighting. Though I might have to talk to someone to see if they can cook for me – I’m useless.” The UFC, which reports that the sport is broadcast to 800 million households worldwide, has been paying Calderwood not to fight, to avoid injury before the show, and she is hungry to compete. Which of the housemates does she most want to take on? “All of them.”

Until recently her coach and manager was her boyfriend James Doolan, a fellow fighter in the Griphouse’s Dinky Ninja Fight Team; but she is in London today (on the eve of an MMA showcase at the O2 arena) to meet her new professional management and the UFC bigwigs. She and Doolan live together in Glasgow with a fridge full of low-calorie raspberry jelly (an essential treat while “cutting weight”) and a cupboard full of Jaffa Cakes, which are the only solids allowed in the early stages of rehydration, after the dieting but before the action. The quick, painful 10lb weight loss before a weigh-in, assisted by fasting for 24 hours (with sauna and salt bath), is a process the couple gently steer each other through, both of them gripped by a benign monomania which means they talk of little except their sport. “We are both totally obsessed,” says Calderwood, who never has a holiday or even a night out (except Nando’s after training on a Friday) and rarely buys clothes apart from kit.

I ask for her sweetest moment so far. She looks dreamy and says they have all been “pretty perfect” but chooses her second MMA fight in May 2012 against Lena “Hunter” Ovchynnikova of Ukraine. “She was the promotion’s golden girl and I was just starting out; they were bringing me over to be fed to the wolf but I slayed the wolf. I beat her and I totally dominated her on the ground, which no one had seen or expected.” Despite strict rules banning hair-pulling, eye-gouging and so on (the sport has more than 32 fouls), Calderwood claims to have been fouled in that fight, which made her underdog victory especially precious. “She bit me on the arm and tried to headbutt me. She had her gumshield in but you could see the mark of her bottom teeth.” Impressed with the new contender, Invicta signed her up. Later that year Calderwood left Ashley “Smashley” Cummins, a St Louis County police officer, writhing in agony with a mean right knee to the face. “I think she broke her eye socket… I was worried she was going to arrest me.”

Calderwood’s third MMA fight, against French fighter Ainara Mota, Glasgow, June 2012

Even though she wears no helmet, just fingerless gloves and a gumshield for protection, her own worst injuries have been an accidental black eye and bruises to her shins. The British Medical Association might disapprove of her sport but she is physically unafraid, claiming that MMA’s safety rules – a knockout stops the fight, referees call a halt if one fighter is not defending themself – make it safer than boxing. “I don’t mind getting hurt but what scares me is going out there and disappointing my teammates, my family, by not performing. This is what I’m good at and I’m proving it every time to myself and them.”

And to the tartan army, of course. Scotland’s first professional female MMA artist carries the St Andrew’s cross into the octagon. When she fought the Austrian Livia von Plettenberg in January 2013 she wore a tartan micro-kilt and, with her braids and punches flying wildly, looked like a post-apocalyptic punk warrior, a nightmare spectre, and also somehow a wholesome homegrown lassie giving it her all. “But the kilt isn’t the perfect garment for being on the ground,” she smiles. “I just knew that one was going to be a stand-up fight, and I was right.”

MMA is a huge cash-spinning operation in the US, Canada and Brazil, and a big celebrity business. But Calderwood barely tolerates, let alone pursues, the idea of fame; she dislikes even speaking up to defend her sport against those who would like to see it banned. Before her first MMA fight in 2012 at Glasgow’s famous Kelvin Hall, against Noellie Molina of France, local member of the Scottish parliament Sandra White pronounced the match “degrading to women” but Calderwood, who pummelled “whatshername… Napoleon” with ruthless efficiency, refused to waste energy debating. Asked about the sexism of the sport, the bikini-clad UFC “ring girls”, she shrugs. “It’s giving someone a job isn’t it?”

Perhaps as a result of avoiding negativity, she seems deeply calm and self-contained, protected by her close relationships and the cosily safe world she has constructed around her dangerous occupation, taking her own sleeping bag to even the smartest hotels for the smells of home, calling her mum every night. She has never consulted psychologists for mental preparation: for motivation she might look at the online Quote Garden. She reaches for her iPhone and reads aloud her current thought for the day: “Do epic shit. Go forth and be a force of awesome.”

Calderwood thinks she has five years of competition left, and a whole lifetime of ambition to pack in. “I will keep going until my body gives up.” For now, there is a sunny day in London to fill and maybe she’ll ride on the London Eye: is it expensive, she wants to know? But this is small talk when what she really likes to dissect, and with those who know their stuff, is her sport. None of my questions can compete with the one she was asked by a fan at a Q&A session the previous night. Would she like to be the first woman to knock someone out with a kick? Obviously. And what tips could she offer for effective street fighting? She had avoided answering this but when I press her, the baby-faced killer laughs. “My tip is to run a mile… especially if it’s a drunken girl chasing you.”

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