Xander walked away from Ethan's feeling quite lucky. Not only had he found a 20 while making his way to the store, after the owner Ethan learned he knew his old buddy Giles, the man had sold him a whole box of Naruto themed accessories and props at a bargain price, mostly because something had spilled on the box' content. While it wasn't a costume, he could use the box with his army fatigues to create his own ninja.
Too bad he hadn't been able to convince either Willow or Buffy to wear Naruto themed costumes as well.
As Xander went through the box, pulled out kunai, shuriken, headbands with reversed village symbols, scrolls of various descriptions and supposed purposes. It was when he found several sets of doujutsu lenses that he became excited. Because as much as he loved the main character Naruto, Xander was at heart a sharingan fanboy.
His character would be Uchiha Ryu (dragon in japanese), a talented but very unlucky member of the Uchiha clan. He awakened his mangekyou sharingan while being forced to witness the brutal torture, rape and eventual murder of his teammate (and not so secret crush) Uzumaki Kasumi, during one particularly hellish mission as chuunin.
Their assailants' deaths were equally painful, being slowly burned alive by the unrelenting flames of amaterasu. The leader of the enemy group survived by severing his own arm, and then stabbing him through the heart just as his own teacher arrived, to create an opportunity to flee.
This forced his teacher to perform an emergency heart transplant, replacing his heart with poor Kasumi's heart. Aside from the gift of life, Kasumi's heart came with Uzumaki chakra, expanding chakra reserves and increasing vitality, to the point he could be considered an Uzumaki runt.
Years later on a mission with Ryu as jounin team leader, Ryu was forced to kill his elder brother Arashi when he became controlled by an enemy nin. Knowing his brother wouldn't mind, given the circumstances, Ryu took Arashi's eyes for himself, and awakened the eternal mangekyou sharingan. Invoking 'plot device no jutsu', Ryu's looked exactly like Madara's had, because Xander had no other options among the lenses.
Where Ryu had been able to use all three of the standard mangekyou jutsu tsukuyomi, amaterasu and an incomplete susano'o, his brother Arashi's mangekyou had granted him perfect understanding of his own energies and the ability to combine dual chakra natures without needing a bloodline for it. Especially fire-nature chakra combinations (such as boil, lava and scorch release) came easily, because of the mangekyou's amaterasu fire nature.
After awakening his eternal mangekyou sharingan, those abilities grew stronger as well. His control over tsukuyomi, amaterasu and susano'o grew considerably, with Full Body mode becoming available for the latter. He quickly grew able to sense natural energy, which gave him a considerable edge when he began sage training. He also gained the ability to combine up to three chakra natures (and recreated dust release) as well learning how to use yin-yang release.
It was during the fight against Edo Tensei Madara and the Juubi that he used a yin-yang release infused izanagi, sacrificing his left eye to undo his (rather messy) death at Juubi's paw. It restored him to his prime rather than 'simply' revive him, but with only one eye left, he was no match for Madara. Not much later in that very same battle however, he awakened the rinnegan, restoring his left eye in the process. Unlike Edo Madara's rinnegan, his were a vibrant green emerald, with 6 tomoe, like Sasuke's.
Quite instinctively he mastered the rinnegan's abilities, many being more powerful versions of his earlier powers. Madara's Edo rinnegan couldn't compete with a pair of true rinnegan on the field, and was quickly separated from the immature juubi, which in turn was absorbed within Ryu as he became its jinchuuriki. Intercepting Black Zetsu's attack from the back and killing the thing, Ryu inadvertently and unknowingly derailed Kaguya's resurrection plans.
As Xander had no rinne sharingan lens to glue to his forehead, he simply drew on a closed third eyelid, imagining it concealed a blue rinne sharingan that had yet to awaken but would soon enough.
After a night filled with dreams of being a ninja, Xander woke feeling oddly more refreshed than otherwise. His basement 'room' looked much brighter too, for some reason.
That reason became very obvious when he saw his reflection in the mirror; his eyes were mature sharingan! Xander knew immediately they were real, because his reflection moved in slow motion where he knew he was moving normally. Also, he hadn't worn sharingan lenses that night, he'd worn the green tomoe-ed rinnegan variation...
He stilled completely as he considered that little fact. Not quite sure how, he willed his sharingan to 'power up' and was rewarded with the emergence of Madara's eternal mangekyou sharingan seal.
"Oh fuck..." Xander breathed. He could feel his eyes weren't at their most powerful yet. Moments later he was observing himself through a pair of rinnegan eyes.