Yery stands thirtieth and writes the vowel that Slavists transcribe as y, a high back unrounded vowel still present in Russian, Polish and several other Slavic languages. It carries no numeral value.
It is not a single sign in the early alphabet. It is written as a compound, the back jer followed by an i, and the two elements stayed visibly separate in the manuscripts long enough that alphabet lists disagree about whether to count it as a letter at all. Some manuscripts use izhe rather than the decimal i as the second element. Glagolitic writes the same vowel as a compound too, so what the two scripts share here is a strategy rather than a letter.
The compound spelling is not arbitrary. It encodes an analysis: this vowel is what an i becomes after an unsoftened consonant, and the spelling says so by writing the mark of unsoftening in front of the i. Whether that analysis is correct is still argued in modern linguistics, which is the subject of the companion article.
The letter survives in Russian, Belarusian and several other modern alphabets, still written as a compound shape, still doing the same work. In Ukrainian the corresponding letter has a different shape and a different history.