JyoudaoicibaDictYouDict[J 词源字典]
the letter is a late modification of Roman -i-, originally a scribal creation in continental Medieval Latin to distinguish small -i- in cursive writing from the strokes of other letters, especially in the final positions of words. But in English, -y- was used for this, and -j- was introduced into English c. 1600-1640 to take up the consonantal sound that had evolved from -i- since Late Latin times. This usage first was attested in Spanish, where it was in place before 1600. English dictionaries continued to lump together words beginning in -i- and -j- until 19c., and -j- formerly was skipped when letters were used to express serial order.

Used in modern writing to represent Latin -i- before -a-, -e-, -o-, -u- in the same syllable, which in Latin was sounded as the consonant in Modern English you, yam, etc., but the custom is controversial among Latinists:
The character J, j, which represents the letter sound in some school-books, is an invention of the seventeenth century, and is not found in MSS., nor in the best texts of the Latin authors. [Lewis]
In words from Hebrew, -j- represents yodh, which was equivalent to English consonantal y (as in hallelujah) but in many names it later was conformed in sound to the -j- of Latin or French (Jesus).[J etymology, J origin, 英语词源]