Psi is the twenty fourth letter of the Coptic alphabet and carries the numeral value seven hundred; Coptic Orthodox usage names it epsi. Like ksi it writes a cluster of two consonants rather than one, standing for p followed by s, and like ksi it is optional, since Coptic may spell the two letters out instead. That makes psi one of the rarer characters on a Coptic page in its phonetic role, while its numeral duty at seven hundred is entirely steady. It belongs with ksi, theta, phi and khi in a group of Greek letters whose usefulness to Egyptian was never obvious, and what Coptic did with that group is one of the more revealing things about how the script was settled.
The tradition did not prune the Greek alphabet down to the letters Egyptian needed. It kept the series whole, complete with letters that write clusters, letters for sounds Egyptian lacked, and three signs that write nothing at all and exist only for counting, and it then added seven letters of its own at the end. An alphabet assembled that way records its history in its shape, and a Coptic alphabet chart read from left to right is a summary of where the script came from. There is an instructive contrast here with alphabets that were designed rather than assembled. Glagolitic, devised in the ninth century for a specific mission, has a letter for each sound the language needed and very little that is redundant, because one person settled it at one time for one purpose. Coptic looks nothing like that, and the difference is not a matter of quality. An assembled alphabet carries its history in its irregularities, and a designed one does not.