Psi is the penultimate letter of the Greek derived part of the Coptic alphabet, with only omega after it, and it is a convenient place to take stock of what that part contains. Twenty four letters came from Greek, in the Greek order, with the Greek numeral values, and a twenty fifth sign, sou, was inserted at the sixth position for counting alone. Of those twenty five, some went straight into heavy native use because Egyptian had the sounds: alpha, ei, eta, iota, kappa, laula, me, ne, ou, pi, ro, semma, tau, omega. Some found almost no native work and live in loanwords: gamma, delta, zeta. Some write clusters and are optional: ksi, psi.
Some double as compressions with the Egyptian letter hori: theta, phi, khi. And one, sou, writes nothing. After omega the alphabet changes character entirely, because the next seven letters are Egyptian, taken from Demotic, carrying Egyptian names and writing Egyptian sounds that no Greek letter could reach, and carrying no numeral value because they were never part of the Greek series the values came from. That break in the middle of the alphabet chart is the whole story of the script in one line, and it is visible to anyone who looks at the chart, although nothing on the chart itself announces it.