There used to be only 25 letters in our "Modern Roman" alphabet. Which letter was added later, and when?

May I recommend this site from the University of Maryland, which shows the way that a number of alphabets have evolved over time. It's really quite cool.

http://webspace.ship.edu/cgboer/evolalpha.html

As for the standard 26 letter alphabet used in English, I'm pretty sure the newest letter is the letter J. Both vowel i and consonant j were represented by I until the 15th century. Then some of the monks working on illustrated manuscripts began to lengthen the I and curve it left when it began a word. Gradually this came to represent the consonant sound with the old form used for the vowel sound. The consonant sound is originally thought to have been like our y (so Julius was pronounced Yulius). The dzh sound came from Old French which has since softened in modern French.

It is W, which replaced a Runic Wynn with a double Latin VV in the 1300, following a U and J also added later than the others.

https://www.quora.com/Which-are-the-newest-letters-added-to-the-English-alphabet