[@Ellipses] I couldn't really find a place in the backstory to fit it without it feeling kinda out of place, overly specific or self aggrandizing- and it didn't seem like detail that deserved an entire paragraph on its own. The tattoo would be something I would've expanded on during a post or something, but Joe'shwa got his main tattoo on his arm sometime shortly after he bonded with his Eidolon. The tattoo itself is of a dangerous mythical creature, but is supposed to both protect him and guide his aim- kind of symbolizing how Eidolons, while similar to the dangerous monsters that roam the world are essentially like protective spirits for those they bond to. 

I also like to think that Raijū sits on Joe'shwa's shoulder around where the tail of the serpent is, and the electricity he channels runs through the length of the tattoo before culminating at the arrow.