'It just was a funny accident."
When Lauren Child first introduced Ruby Redfort as the fictional Clarice Bean's favourite literary character in 2002's Utterly Me, Clarice Bean, she had no intention of spinning the teenage spy off into her own series.
"I'd been writing the Clarice Bean books, which were getting to the stage where they were almost becoming like graphic novels because there were so many words and pictures," she recalls.
"So I started writing these chapter links, and it was difficult at first to know how to structure them, because I'd never written straight prose fiction before. That's where Ruby Redfort came in, as a way of structuring the books, by having excerpts from Ruby's world and her life that Clarice can then use and think about in her world."
Child then got letters from readers asking where they could buy the Ruby Redfort novels.
"There were people who thought they were real books and other people who thought that I should write the books," says Child, who initially found the step-up to an intricately planned series of novels quite daunting.
"The Clarice Bean books are written in the first person and are very anecdotal, which comes easily to me," she continues. "Writing the Ruby books was quite tough because of the genre, as was writing something so finely plotted in that every single thing had to play out.
"I had to keep the thriller side of it going by constantly coming up with these moments of jeopardy, and I also had to make it funny as well."
But five years after the release of the first Ruby Redfort novel, Look Into My Eyes, the London-based author and illustrator brought her adventures to a close with last year's Blink and You Die (paperback edition now out).
After Child launched her career with the extensively illustrated I Want a Pet and Clarice Bean, That's Me, I Will Never Not Eat a Tomato, she released the first volume in her best-selling series for toddlers, Charlie and Lola.
However, the Ruby Redfort books featured virtually no artwork until Child included some character portraits in the back of Blink and You Die.
"There've just been a few diagrams and that's it," she says. "I'd originally intended to illustrate all six books but it was partly because they took so long to write that illustrating them as well became an idea of lunacy, so I decided not to do that, although I'd always wanted to do some for the final book.
"I'm a big believer in picture books, and I love them, but that's the advantage of not illustrating it," says Child.
"You can decide for yourself whatever you want the characters to be, and I really noticed that with Ruby, who is the best described character in them. I describe her quite clearly as having brown hair and green eyes but I still get girls coming up to me saying 'I think she's blonde with brown eyes'. What's so nice is that even now the readers still want to put themselves into the book."
While Child concedes that Ruby was initially there as a foil for Clarice, the teenage secret agent has since come into her own in a genre previously dominated by male protagonists such as Anthony Horowitz's Alex Rider and Charlie Higson and Steve Cole's Young Bond.
"I definitely wanted to write about a very strong girl and I wanted to put her up front, as girls are usually the sidekicks," says Child. "I'm rather tired of watching children's films where the girl can be sassy but is not allowed to be funny, so I thought I'd write about a female character who is allowed to be the lead and who has a boy sidekick."
She also wanted to weave some intriguing puzzles into the storylines which she has developed in partnership with the mathematician and author, Marcus du Sautoy.
"As soon as I realised that [the books] had to be complicated, I realised that they were going to be very difficult for me to write because coming up with codes is not my sort of thing. I was slightly kicking myself for having written that into the bits with Ruby in the Clarice Bean books and I was thinking about dropping it."
But Child decided that would have been slightly cowardly, so she talked to her publishers and they came up with the idea of asking du Sautoy, who's passionate about getting children interested in maths, to help. He was only too keen to assist.
Now with Blink and You Die, Child has resolved to conclude Ruby's perilous missions - at least for now.
"I was commissioned to do a six-book series and I'd decided that it was going to be an interlocking world and a continuous plot," she says. "I based the different books around the six senses, so that's been the arc of it. But it's also got a lot to do with the colour spectrum as well, and my publisher suggested I could maybe do another couple and do all the colours."
However, Child declined, saying it was too hard to keep a plot going and it risked becoming repetitive and unwieldy.
"Funnily enough, the final book was probably the easiest one to write because you're just tying everything up, which is much easier to do that than just keeping it going."
* Lauren Child speaks at the Auckland Writers Festival on Saturday, May 20 from 9am to 10am at the ASB Theatre of the Aotea Centre and takes part in Sunday's Family Day.