This is the long front e, the same tongue position and the same absence of lip rounding as its short partner, sustained. The pair sits side by side in the alphabet, short first, as every vowel pair here does. Every vowel in this alphabet is a letter in its own right, standing on the line and taking its turn between the consonants. That is not how the writing it grew out of worked, where a reader supplied much of the vowelling from knowledge of the word, and it is not how the Indic scripts of the same centuries worked either, where vowels hung as marks above and below a consonant sign. Avestan writes them out. It stands tenth in the alphabet as a whole and tenth of the sixteen vowel letters, between Avestan E and Avestan O. The order is the work of the people who devised the alphabet rather than an inheritance from anywhere, and reading the list straight through recovers the analysis they were working from.
Comparison makes the point quickly. Book Pahlavi, the hand this alphabet was drawn from, had let so many sounds converge on shared shapes that reading it depends heavily on already knowing the word. The Avestan alphabet takes that apart and assigns one sign to each sound, which is why it needs fifty-four of them. A reciter using this alphabet is not being taught the text. The priesthood that used it already knew the Avesta by heart, and the page served as a check on memory rather than as a substitute for it, which is why the script records fine detail a learner would never need.