Izhe stands tenth, writes the vowel i, and carries the numeral 8 from Greek eta. Its name is the Slavonic relative pronoun, the word for who or which, and it is one of the most frequent words in the liturgical texts the alphabet was built to carry.
The mismatch between its shape and its sound is a fossil of Greek. The letter is Greek eta, which in classical Greek wrote a long e, but Byzantine Greek had merged that vowel into i centuries before the mission to the Slavs, and it is the Byzantine value that Cyrillic took. The numeral came with the shape, so this letter carries 8 while standing tenth in the alphabet.
In the Russian tradition it acquired a name built on that number. It is called the octal i, to distinguish it from the following letter, called the decimal i, and the two are told apart in speech by their numeral values rather than by anything about their sound. It is a rare case of an alphabet naming its letters after arithmetic.
The letter survives in every modern Cyrillic alphabet as the ordinary sign for i, and its printed shape has been mirrored relative to the Greek eta it descends from, a change that took place gradually in the manuscript centuries and was fixed by the first printers.