If the real-life Romeo earns enough "boyfriend power" points—by completing game tasks like homework or exercise to become smarter and more buff—the reward is a virtual trip to Atami.
In the game, the couple tours the local landmarks. The girlfriend changes into a yukata, a casual summer kimono, to go see the fireworks, and then they stay overnight at the Hotel Ohnoya. It is known for its cavernous, white-columned baths in the style of Ancient Rome.
In his first visit to the real-life Atami, Love Plus+ gamer Shunsuke Kato planned to walk around the city and see the sights familiar to him from playing the game. One small hitch: his girlfriend, Manaka, was giving him the silent treatment.
She was upset that he had been so busy at work that he had been playing the game only 10 minutes a day. "On days off, I spend one to two hours with her. I guess, compared to the people who come here, our relationship is a bit lukewarm," said Mr. Kato.
Located at the bend of the Japanese archipelago and a one-hour train ride from Tokyo, Atami has definitely seen better days.
The number of overnight visitors has dropped by half from the peak in the late 1960s. The main shopping area has many boarded-up storefronts—a lot of them defunct bars, clubs and other remnants of the city's heyday as an entertainment mecca.
The city is going all-out to indulge ardent Love Plus+ fans.
At the real Hotel Ohnoya, which opened its doors in 1937, the staff is trained to check in Love Plus+ customers as couples even if there is only one actual guest. Says Atsurou Ohno, the hotel's managing director, "We try not to ask too many questions because we want them to be able to remain immersed in that game world."
Some devoted fans will go so far as to pay twice the rate—most hotels in Japan charge per guest not per room—to indulge the fantasy that they are not there alone. A night's stay, at most, can cost $500 though many rooms are cheaper.
In Atami, the Love Plus+ fans—mostly men in their twenties and thirties—stand out. Unlike the deeply tanned beach crowd wearing very little, they are often pasty and overdressed for the heat in heavy jeans and button-down shirts.
Tatsuya Fukazawa, a 19-year-old college student, was visiting Atami for the first time on a recent weekend. In a small waist bag, he carried his Nintendo DS. Once he turned on the device, his virtual girlfriend Manaka Takane—a Libra who enjoys making pastries—greeted him in a syrupy sweet voice.
"There isn't a lot of romance in my life and this helps me cope with some of the loneliness," said Mr. Fukazawa with a chuckle.
Adding real elements to the virtual relationships is central to the Love Plus+ series. The games can be synched up to an actual calendar and clock, which means playing the game too late at night might mean that the virtual girlfriend is already asleep. Players are expected to remember important dates like birthdays and holidays.
Local businesses are feeling the love.
Yamadaya, a shop selling processed fish cakes on Atami's main shopping street, started offering special Love Plus+ fish cakes at the end of July. On top of a rubbery square white cake about the size of piece of toast, Yamadaya draws the characters' faces using black squid ink. At 450 yen ($5.30), the store has been selling out its daily allotment of 150 cakes—50 for each girl.
Korean barbecue-inspired restaurant Hien says a quarter of its customers are currently game-related. For 5,000 yen, customers get a special Love Plus+ menu of Japanese beef and side dishes.
Kanji Nagasawa, Hien's owner, says he is accustomed to making small talk with customers, but the Love Plus+ crowd often sits in silence and plays the game while eating.
"We've been stunned," Mr. Nagasawa said, "how happy this makes those customers."
Alas, the boom ends this month, when the imaginary characters have to go back to school.
Write to Daisuke Wakabayashi at Daisuke.Wakabayashi@wsj.com