Ϙ

Koppa or Qoppa (Ϙ, ϙ; as a modern numeral sign: ) is a letter that was used in early forms of the Greek alphabet, derived from Phoenician qoph . It was originally used to denote the /k/ sound, but dropped out of use as an alphabetic character in favor of Kappa (Κ). It has remained in use as a numeral symbol (90) in the system of Greek numerals, although with a modified shape. Koppa is the source of Latin Q, as well as the Cyrillic numeral sign of the same name (Koppa).

In Phoenician, qoph was pronounced [q]; in Greek, which lacked such a sound, it was instead used for /k/ before back vowels Ο, Υ and Ω. In this function, it was borrowed into the Italic alphabets and ultimately into Latin. However, as the sound /k/ had two redundant spellings, koppa was eventually replaced by kappa (Κ) in Greek. It remained in use as a letter in some Doric regions into the 5th century BC.

...
Wikipedia