Ti is the thirty second and last letter of the Coptic alphabet, the last of the seven taken from Demotic, and it carries no numeral value. It is unusual among the letters of any alphabet in that it does not write a single sound but a whole syllable, standing for t followed by i. That makes it a survival, inside a fully alphabetic system, of the older Egyptian way of writing in which a sign could carry a group of sounds rather than one.
Coptic can also spell the syllable out with tau and iota, so ti is optional in the same way that theta, phi, khi, ksi and psi are optional, and manuscripts differ over which they prefer. It is nevertheless common, and it does grammatical work, appearing in the first person singular verbal prefix among other places, so a Coptic text of any length will use it. Its position at the very end of the alphabet, after six other Egyptian letters that themselves come after the entire Greek series, is a fair summary of how the Coptic script was put together. Nobody designed it. Greek speakers and Egyptian speakers in Roman Egypt took an alphabet that was to hand, kept all of it including the parts that did not fit, added what the language could not do without, and stopped when the writing worked.