Virtual friends have become a fad among lonely or socially awkward adolescents. Over 4 million people have downloaded an app allowing users to chat with an imaginary friend on their mobile device.

App developer Jang Tae-gwan said 70 to 80 percent of people who downloaded the app are in their teens. The app was designed to send a long comforting response to sentences such as "I'm lonely" or "I'm depressed."

Psychologists say the app's popularity reflects the difficulties young people experience in establishing and maintaining relationship with real human beings.

In a survey by Seoul National University in February, 58.2 percent of middle schoolers said they have unstable relationship with classmates, and one out of five were at risk of being bullied.

One youngster who enjoyed chatting with an imaginary friend said, "I would like to chat with my real friends on KakaoTalk (a messenger service app), but I can't because I'm all by myself."

Prof. Hwang Sang-min of Yonsei University, said, "People who feel lonely are desperate to find someone who sympathizes with them and reacts to what they say. It doesn't matter who talks to them -- they just want to hear something from anyone."