About: Riddler wannabe.
Question: Together we are 51, but you will find us all in 26. While we often appear alone, it's more common for us to mix. If in order, we 5 span from 1 to 21. We do not come in a row, for each of us skips a few places. We are all unique but none of us have faces.
Answer: Vowels. If you assign each letter of the alphabet a numerical value, the vowels add up to be 51, and their positions lie in the span of 1 to 21.