There are 6 elements in total: Fire, Water, Wind, Lightning, Light and Dark.
Fafnir boosts Fire and Water, so, if you plan to main Fire or Water, Fafnir is a decent choice if you don't get a good SR princess.
And I am just regurgitating information from Japanese wiki others have made. I am still learning this game myself! Heck, after answering questions, it helped me to figure out which SR Princess I want, so this is to my benefit as well. So I doubt I have what it takes to write FAQs or similar.