A blood-sucking "friend" that probably wants to kill you.
You can suck entities blood which can then be turned into hunger... yummy.
You can turn into a bat and fly about like the little bat guy you are. (Or bat girl - etc)
Gain abilities at night time
Other players can craft a wooden stake dealing big amounts of damage to you
You burn in daylight if not wearing a helmet
You can negative effects and die if on low blood