Blood Blade (Oma Sei)

From Kodansha:
Having fallen in battle long ago, Count Vlad Dracula is reborn as a katana-wielding young vampiress in an alternate-history Europe. After the reincarnated Dracula rescues a girl named Clara from a mysterious stranger, Clara explains that she is the creation of a certain Victor Frankenstein. She also reveals that she is fleeing from an organization called Cerberus, which seeks to capture and study “monsters” such as herself in order to create an army of human-monster hybrids. Clara begs the vampiress to flee with her to an island of monsters where the two of them will be safe but as they set out for Monster Island, the sinister forces of Cerberus are in hot pursuit.