A mystery keyword searcher is looking for:

a book about a girl called leslie who likes tattoos and falls in love with a demon

I love it when I know the answer! The book is Ink Exchange by Melissa Marr, the second book in her Wicked Lovely series. It’s a young adult urban fantasy series, but lovers of urban fantasy and dark fantasy of whatever age should really enjoy it. However, the tone is different than it is in Wicked Lovely. Ink Exchange is extremely dark and even horrifying, so readers looking for a standard paranormal romance with a happily-ever-after will probably prefer something else.