Brother and sister Shin and Yuko Kibayashi are the creators of the best-selling Japanese manga Kami no Shizuku or Les Gouttes de Dieu (Drops of God), which appears in the weekly manga magazine, Morning. The story centres on Shizuku Kanzaki whose father, a world famous wine critic, dies suddenly. He leaves a vast wine collection but in order for the young Shizuku to claim his legacy, he must first discover the identity of a group of wines, the “12 apostles” ­mentioned in his father’s will. A novice in the wine world, he is pitted against Issei Tomine, his father’s adopted son (and rising wine connoisseur), but helped by trainee sommelier Miyabi Shinohara.

Since it started in 2004, four million copies have been sold in Japan and the manga is now being translated and sold in Hong Kong, Taiwan, France and Korea, where it has sold 2.5 million copies. The Kibayashis write under the pen name Tadashi Agi and work from an office in Shin’s home in Kichijoji, Tokyo.

“We’ve been writing manga together for 15 years so we’re always thinking about possible stories. When we work, we exchange ideas and drink a glass of wine or two. We weren’t always interested in wine – we’d happily drink beer or sake – but a few years ago, we threw a party and opened a fantastic 1985 Romanée Conti Echézeaux. That was it – from then on we became wine maniacs.

In those days, we were working on another manga, Psycho Doctor, about a psychologist, with Shu Okimoto, the artist who draws Kami no Shizuku. One day we started talking about the images conjured up by a particular wine. One of us said: ‘Doesn’t this wine remind you of a woman with black hair?’

And then we just ran with the idea. We realised that there was a manga in there but our publisher, Hirokawa-san, wasn’t so enthusiastic. The wine market in Japan is a particular niche – it’s still not something the average person drinks and at that time wine was regarded as difficult to ­understand. He thought it was risky, but we persisted and the first issue came out in November 2004. In our third story we mentioned Château Mont-Pérat, saying that it was delicious but reasonably priced. Next thing the wine was being discussed all over the internet and shops started selling quantities of Mont-Pérat. We realised we were onto something.

We write for one or two days a week. First, we drink – and sometimes we drink too much to work, to be honest. While we’re chatting about a wine, the idea gradually takes shape. Once we get a rough outline, it’s easy. The hard bit is choosing the wine. Some wines don’t lend themselves to a story, even the great wines. Take the ­Romanée Conti 1985 – it’s so complete, it’s hard to find a character in there. It’s too perfect for our purposes. Chevalier-Montrachet worked well – it has a sharpness to it. We compared it to the Matterhorn – a tough climb but once you get to the top you appreciate the scenery.

Sometimes we reminisce about our childhood. There used to be a famous French restaurant in this neighbourhood with a proper French chef, which was really something in those days. Our grandfather, who was a real gourmet, used to take us there. We can still remember the taste of the double consommé and the coq au vin.

While we’re talking we usually come up with three or four stories and one of them works. It helps that Okimoto-san [the artist] understands wine so well. We send him the stories. We’ve written six out of 12 apostles, so we’re halfway through. We think there are at least two more years to go.

We never anticipated the influence we’d have on the wine market. We just wanted to talk about wine in an everyday way. The general public doesn’t relate to the language wine critics use. Who ­understands if you say a wine tastes like a ‘wet ash tray’? Now if we mention a wine in the manga, it sells out. We have to be careful when we include wines we like personally. We usually say that what we drink depends on mood and the season.

We come into contact with a lot of people from the wine industry these days. They come to the house to see us – winemakers from France and Italy, Japanese importers and wine sellers from Australia and the US. Maybe they hope we’ll ­include the wine in the manga but we’d never write about a wine just because it was given to us – we’d be betraying our readers. Everything we recommend we pay for from our own wallets. We go to a lot of wine tastings and must have tried 10,000 wines by now. ANA asked us to its wine tasting to choose wines for its first-class cabin.

We throw wine parties three times a year and invite 30 wine experts and wine lovers. We usually try about 40 kinds. We have a cellar downstairs and another apartment nearby, which we’ve turned into a cellar. Even the bathtub there is full of wine bottles.

While we’ve been writing the manga, wine sales have gone up in Japan and more people are interested in taking wine courses. Japanese wine is still seen as a curiosity in the industry. There are only a couple of places in Japan with a suitable climate for wine – Nagano and Yamanashi – but the wines are getting better and a few of them are good, such as Grace, which comes from Katsunuma.

The manga was turned into a TV drama in Japan this spring; we consulted on the wines but everything else – the actors, even the story – was left to the producers. It turned out quite different from our manga. It’s being made into a series in Korea next year and the French version of the manga sells well so we are hoping that it might be turned into a film. Now, we’d love to have the manga translated into English.”