Danny has an ever evolving tattoo on her upper left thigh with hash marks divided by more hash marks.

…Thirty-six over 154. He knew he shouldn’t, but it was irresistible. He took one hand off the wheel and traced a finger over the fabric of her pants where he knew the tattoo was hidden. “What are you counting?”
She didn’t answer.
Yenya games?”
She scoffed.
She laughed. “Not even close. Move your hand before I—”
“Before you what?” he glanced at her for a second then back at the road as he traced more hash marks on her thigh.
“Now who’s playing with fire…”

-Esper’s Touch

Click here if you've already read Esper's Touch (spoiler if you haven't)
She’s keeping track of lives lost and lives saved.