The Bloodsucker is a creature resembling three leeches fused together at the base, which writhes like octopus tentacles. It appears in two places: the Otherworld version of Alchemilla Hospital and Nowhere. In the hospital, it is found in Room 204, sucking at a puddle of blood. It becomes aggressive once Harry Mason approaches it, so he must distract it in order to reach the “Plate of Hatter” behind it. This can be done by using the blood pack previously found in the storage room.
In Nowhere, it is the creature living in the fridge in the kitchen; the first door to the right after using the Key of Phaleg. After retrieving the Dagger of Melchior, the player has to use the Ring of Contract to fix the chain on the fridge, keeping the creature from breaking out and killing Harry. Unlike the other monsters in the game, Harry cannot kill the Bloodsucker.
The Bloodsucker is and acts as, essentially, a colossal leech. The parasite is fueled by the desire to fill itself, and therefore has an extremely strong magnetism towards sources of blood. Like many organisms, it is defensive over its "prey", and will therefore respond violently if Harry attempts to approach either the blood pack or the puddle of blood it had been previously eating. Its need to sustain itself with blood is also clear in Nowhere when it attempts to break free from the fridge and galvanize Harry, hoping to feed on his blood. However, seeing as the Bloodsucker in the Otherworld of Alchemilla Hospital never averted its attention from its available meal to focus on Harry (unless Harry's proximity became too close for it to tolerate), it appears likely that it takes on scavenger behaviors and also hints that there must have been no source of blood in the fridge during its latter appearance. The Bloodsucker, overall, is rather nonchalant towards Harry unless disturbed while feasting.
The Bloodsucker serves as the embodiment of Alessa Gillespie's strong aversion to leeches, worms, snakes and other similar entities. It's also very probable that the creature could hold symbolism of being "sucked in" to something, for both Harry and Alessa. Harry Mason was unintentionally dragged into the business of the Order, and towards the end of the game, Dahlia Gillespie even says her daughter should be thankful for the man's inadvertent assistance, putting him into a position Harry never endeavored to occupy. Alessa herself was also "sucked into" her mother's plans; albeit a believer in the Order, she never wanted part in the heinous ceremony her mother performed on her.
The beast is also very likely to symbolize greed, as seen with its ravenous appetite for blood. This selfishness could be a hint towards Dahlia, seeing as she was practically blind with yearning for her daughter to hasten the birth of the Order's God.