DEMON SACRED (Demon Sakurido), by Natsumi Itsuki. First published in 2003 and first published in North America in 2010.
PLOT:
Fifteen years ago, strange phenomenon started to occur across the earth. Demons straight out of mythology appeared, and a condition known as Return Syndrome started to retroactively age people back to zygotes. Mona's twin sister Rina is one of those affected and Mona is determined to save her by any means possible. She gets her chance when a mysterious man from their past named Mika returns. Through him, Mona discovers she is heir to an incredible power that allows her to capture and tame the demons. Together, they may be able to reveal the truth and save Rina...if the government doesn't get to them first.
