Iy is the fifth letter of the Samaritan alphabet and carries the numeral value five. It stands where Hebrew has he, and it once wrote the h sound. In the Samaritan reading tradition it does not: it is the second of the four throat letters, with alaf, it and in, whose consonants have weakened, which is why the letter name itself no longer begins with the sound the letter is for.
Weak or not, it does a great deal of grammatical work. Iy prefixed to a noun is the definite article, so it is the letter that turns a word into the word. Attached to the end of a noun it marks the feminine, and attached to the end of a place name it can mark motion towards, the form that turns a name into a direction of travel. It also stands as a vowel letter at the end of a word, one of the small set of consonants that Northwest Semitic scripts press into service for vowels.
Greek took the sign and made it epsilon, a plain vowel, which is the second time in the first five letters that a Semitic consonant sign became a Greek vowel. The pattern is not a coincidence. Greek needed vowel signs, and the letters it had no use for as consonants were exactly the ones available to be repurposed.
The character is encoded at U+0804.