The Greek alphabet letters has been used to write he Greek language since the late 9th or early 8th century BC. This one is derived from the earlier Phoenician alphabet, and was the first alphabetic script in the world to have distinct letters for vowels as well as consonants.
Apparently, Greek alphabet is the ancestor of the Latin and Cyrillic scripts. Just both of them, this one originally had only a single form of each other, it developed the letter case distinction between uppercase and lowercase in parallel with Latin during the newer era sound values and conventional transcriptions for some of the letters differ between Ancient and Modern Greek usage. The main reason behind it is because the pronunciation of Greek has changed significantly between the 5th century BC and today. Both eras also use different diacritics.
Usually, Greek alphabet letters are used as math and science symbols. For those the players of Fortnite who are wondering if these can be used as your username of Fortnite, the answer to the question is yes.
To make the Fornite username using Greek letter, you might need an online Greek keyboard called greek.typeit.org. This one is designed for typing Modern Greek, not the Ancient ones. It is based on the Beta Code keyboard layout, that tries to match Greek characters to QWERTY keys based on sound. For instance, the letter ν (nu), that sounds like n, is typed by pressing N.
To use this keyboard, you can press the key that sounds like the Greek character you want to type. For instance, if you want to type ζ (zeta), you need to press Z. If you want to type accented letters, please type the letters such as υ and then press 1 to add the tonos or 2 to add the dialytika. Aside from that, you are also able to type ύ, ϋ or ΰ, hold Alt and press U once or more than once.
You can press Alt+Enter if you want to turn the Greek layout on and off. To learn its keyboard shortcut, please stop the mouse over each button. Shift+click a button if you want to insert its upper case form. Alt+click a button for those who want to copy a single character to the clipboard. You are able to choose the text and press Ctrl+C if you want to copy it to your document. In the target document, please press Ctrl+V, or for those who want to paste the text without formatting, you can try Ctrl+Shift+V.