Dean got his first tattoo the day the angels fucked off and left him more or less human, an anti-possession symbol that matched Cas's. Since I can't just rely on my natural charm to keep 'em off anymore, he told Cas with a wink as the unsmiling woman covered in sunset roses transferred the design to the freshly shaved patch of skin over his heart. And come on, looks badass, right?
He didn't flinch when the tattoo gun stabbed its ink into his flesh, marked him permanently in a way that might not have been possible when grace flowed through him and healed his vessel. It had stopped being a vessel years ago, even before he stopped being an angel; it was his body now, and his body didn't mind the mild pain. Actually, you know what? I think I kinda like it.
More tattoos followed, when they had time and supplies and someone with a skilled hand. Rarely the same person twice, though there was a stretch of five months between when Joph found Camp Chitaqua and when he got it in the leg from a croat (and then in the head from Cas) when he managed enough sessions to cover most of Dean's back.
Some were practical (the same angel-deterrent warding Cas carried on his ribs, written in black ink above Dean's just in case those fucking dickbags ever come back), some were meaningful (a winged serpent coiling up from his left ankle, curving into an S as it sunk its teeth into his femoral artery; its scales were the same brown-yellow-green Samuel's eyes had been), some were as painful as they were beautiful (for all that Dean shrugged off his fall, his newfound humanity, he still shuddered back a sob every time Cas ran his hands, his mouth over the bloody, torn wings painted across his back), and they were all quintessentially Dean.
Cas only had the one tattoo, but that was fine; Dean liked marking him in other ways. They traded bites and scratches. They fucked like they fought: hard and fast and dirty, hands bruising flesh, bodies slamming into walls as they struggled for dominance. It was the only time Cas still felt alive, when he could feel his blood pumping with something more than anger and pain.
And after, in the tender moments when they'd exhausted their desperation, Cas would kiss his way across every design inked under Dean's skin, tongue tracing the lines, tasting the salt of sweat and sometimes come. There wasn't much left to his heart, but what little there was, he poured onto Dean's flesh. What he left behind might not be as permanent as Dean's tattoos, but they both knew it was more eternal.