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.

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!