It had started as a joke, really. A friend bet him he couldn't go through with a painful human ritual; he mulled it over until he doodled an image of an ouroboros on a 22nd century bar napkin. He liked it so much he decided to get it tattooed on the inside of his arm, that would count as a painful human ritual. The tattoo artist did a really good job so he took the girls business card. Unable to cloak his TT Capsule as a sailing vessel, he had gone with a car. He got in and doodled the snake eating its own tail on the back of the card, sticking it onto one of the monitors where the kitty couldn't get it.
He somehow grew attached to the tattoo, seeing it on his arm everyday, it soon felt like a natural part of himself he was born with. When he regenerated he was disappointed to find he no longer had the tattoo. After a while he felt the need to go back and get the tattoo again. He drew the ouroboros more detailed on a piece of paper, setting his ship's coordinates for the tattoo parlour.
He managed to land only a few months after his first stop there, he asked for Ara, who had done his tattoo before. When she asked how he had found this tattoo, he made up a story about seeing it on some bloke who recommended her. This time he got it on his calf, larger and more detailed.
He always managed to nearly forget over the years that he wasn't born with his tattoo, and one of his first stops after regeneration was always the tattoo parlour for Ara to redo his tattoo, she just didn't feel like herself without the tattoo. She thought perhaps she was still getting used to her female body, but relaxed as she was getting inked, completely at ease when the job was done. She had opted for a more modest size, not even 3cm tall, and placement on her upper thigh. She had suspected it was the tattoo ever since her third regeneration, but this basically proved it. Brand new female body and all she had to do to become comfortable in it was go to Ara and get her tattoo back. Oooh, tattoo back. There was something to consider for next time, she could get it quite large and intricate on her back...
(references info Neil Gaiman gave himself: neil-gaiman dot tumblr dot com slash post/30786184930/eleven-things-you-probably-didnt- know-about-the )