Sanguini is a character who appears briefly in the sixth book of the Harry Potter series. He is a vampire, and the only one who appears in the course of the series.
J.K. Rowling started the Harry Potter series, at least seemingly, as adventure books for children with a random dash of fantasy characters, many of which seemed to appear to be let in mostly for amusement value. Over the course of the series, as it became more serious in tone, some aspects of supernatural horror were depicted with actual dramatic effect, such as Remus Lupin's lycanthropy. But the appearance of a vampire in the sixth book is treated in a much more light-hearted manner. It might have been expected that vampires would be treated as serious adversaries, perhaps siding with Lord Voldemort. Instead, Sanguini is a guest at a party given by Horace Slughorn, where a gaggle of teenage girls look at him, "curious and excited". He is also described as looking like a stereotypical vampire, "tall and emaciated". After being ushered away from the girls and given an appetizer, Sanguini disappears from the action.
The presence of a vampire as a humorous fantasy character instead of as a serious threat could have been written in for several reasons. The first is that at this point, the plot was already confusing enough and full of menaces, and Rowling didn't need to add any more threads of confusion, so she decided to just mention the presence of a vampire as way to fill out the fantasy characters who appear in the series. The other (somewhat theoretical) possibility is that she chose not to address vampires as serious dramatic characters because there is enough books in which that takes place, often to ill effect. In fact, the teenage girls hovering around the vampire looking "curious and excited" could be a somewhat oblique reference to the readership of much vampire pulp.