A dhampir is a child of one vampire parent and one human parent, with all the strengths of vampires, and, save the thirst for blood, few or none of the weaknesses (Alucard, while seen breathing, lacks a reflection and is hurt by water without the right artifact). Though certain sources indicate that even this thirst for blood does not carry to a dhampir child.
According to lore the Dhampir was very useful to the superstitious community because, being part vampire, they can see them for what they are even if they're invisible or had transformed into an animal. A town would hire them to find and kill the vampire, for a fee of course. Much of the time the Dhampir could talk to the vampire and convince them to leave. Some resent their vampiric lineage and do, truly, act as human hunters do and kill the vampire that seems to be causing trouble. The dhampirs that follow this path in life will join gangs of hunters to track down vampires, even their own family in some instances. Naturally, the human hunters are more than pleased at gaining an immortal member with inside information.
A recurrent dhampir character into the game series is Alucard.