Until the Full Moon

Until the Full Moon (FULL MOONにささやいて, Full Moon ni Sasayaite) is a manga series written and illustrated by Sanami Matoh. The series was published in English as by Broccoli Books on February 23 and June 15, 2005 respectively. Kodansha Comics USA picked up the license and republished the series.

Plot
Marlo has a problem. On the night of the full moon, this half-werewolf, half-vampire undergoes a mysterious and terrifying transformation:

He turns into a girl.

Desperate for a cure, his parents call on Doctor Vincent, a long time family friend. But Marlo wants to keep his condition secret from Vincent's son, the vampire playboy David.

Unfortunately, the secret gets out, and an old problem resurfaces: David has been interested in Marlo for years! If a remedy can't be found, their parents believe the next best solution is marriage—a marriage between Marlo and David!