The R1 DVD release for "Nightwalker" treats it as a single, 12-episode TV series. However, the series started as a 4-episode OAV, which is why a there is an ending to the story arc with episode 4. The OAV series was then taken to TV, where an additional 8-episodes were created for the 12-episode count seen here. This is why after "episode 5", the art/animation/character designs are different and why there are no episode previews for episodes 1-4.
"NightWalker - Mayonaka no Tantei" is originally a 1993 16-color PC softcore game for DOS. It was remade and relaunched for Windows in 2001 by the Arieroof company, which later went bankrupt.
A soundtrack was released on October 14, 1998 by Universal Music Japan.
The series aired subtitled on the TV channel Locomotion from 2003 to 2004 in Spain.