Izabella Williams visited her family on October 14, and after returning to London, where she currently lives, she met with a friend in Kilburn on October 16.
She was in contact with her family on October 18, however, she failed to attend a pre-arranged meeting with a friend on October 26.
Officers are concerned for her welfare and are urging Izabella to get in touch to let them know she is safe and well, a Northamptonshire Police spokesman said.
Izabella is white, 5’3’’, with a slight build, brown eyes and shoulder length straight hair that is currently dyed green. She has a tattoo on her left wrist and two mouth piercings.
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Izabella, or anyone who may have seen her, is asked to contact Northamptonshire Police on 101.