Useful details include the person's ethnicity, age or date of birth, and gender, height and build, what their hair is like, any distinguishing marks, a birthmark or scar, any tattoos they might have and where.
Are there other people involved? What are the names or nicknames of the people they know? What is their involvement? Can you tell us their address, including the town, county and postcode. What do they look like? Include their ethnicity, age, sex, height and build. Do you have a phone number for them?