I read this book series in middle school. I believe that it would be classified as young adult fiction/fantasy. A girl has magical powers, and with her growing powers comes runes/tattoos in her arms. This is shown on every book cover. I know that one cover is blue. This one shows the girl in a black tank top and black pants. One is a yellow cover, with her in a dress (I think). The girl has a friend/mentor, who she develops feelings for. The girl is the key to stopping a war between multiple factions/people. Each side is trying to use her to their advantage. I think the series was written in the early 2000s.
Totally just answered my own question. I remembered that the author’s name is Michelle Sagara. Looked up books, and the series is the Elantra series.
These follow the adventures of Private Kaylin Neya, a young woman who serves the Hawks in the towering Halls of Law. She’s Elantra’s version of a police officer, a mortal in a city ruled by Dragons.