TOKYO — In almost every way, Akihiko Kondo is an ordinary Japanese man. He’s pleasant and easy to talk to. He has friends and a steady job and wears a suit and tie to work. There’s just one exception: Mr. Kondo is married to a fictional character. His beloved, Hatsune Miku, is a turquoise-haired, computer-synthesized pop singer who has toured with Lady Gaga and starred in video games. After a decade-long relationship, one that Mr. Kondo says pulled him out of a deep depression, he held a small, unofficial wedding ceremony in Tokyo in 2018. Miku, in the form...