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Beast.Code is a series of japanese LN written by Takahiro Yonemura. The series is about a boy, Raiki, who is among a group of people with junk DNA that is about to be activated into monsters. His girlfriend Tomoko, as it always happens, turns out to be Queen of Beasts and an enemy of mankind.
At some point in the second volume of the series, Diamond Beast, another girl, Reina, is introduced. She can turn into a dragon.
As with most LN information on this series in english is scarce.