Holly is a half-mortal child attending the School for Little Vampires in Therecameaspider's fanfiction, School for Little Vampires, Plus One.
Holly is a half-mortal child, which is a child born between a vampire and a human. The existence of half-mortal children is considered taboo and as a result, there is not a lot of research on them. Count Alarich Von Horrificus allows Holly to attend his school under the condition that he and his staff be allowed to document Holly's development and later profit from their research, even if it takes a century or longer. He also agrees because Holly's mother, a vampire named Morgana, supposedly attended school with Count Von Horrificus and Klaus Pumpkinfiend when they were all children. It is hinted at that Count Von Horrificus may have had feelings for Morgana, at some point.
Holly's father was a mortal man who has been left unnamed. The only information we have is what we learn from Morgana, that Holly's father discovered what Morgana and Holly were some time shortly after Holly was born, and chased them away out of fear. We do not learn how Morgana was able to hide being a vampire from the man she married (or courted, at the very least) long enough for them to have a child together, so we can just assume the man was oblivious. (A good example of such obliviousness would be like Sunshine when it comes to Oskar.)
It is implied in the storytelling that Holly has been aging like a vampire, despite not actually being one; at least, not fully a vampire... yet. During her first night in class, Stoker does his best to make her feel unwelcome and drops a centipede down her shirt and the reaction it causes gets Holly in trouble with Oxblood. Near the end of the story, however, Stoker finds himself at Holly's mercy when Polidori traps all the children. After a tense confrontation between Polidori and Count Von Horrificus, Holly frees the other children but refuses to free Stoker until he apologizes. Which he does, very pitifully.
For the most part, Holly seems to get along with Oksar, Gothetta, and Klott. Leechy and Ashley appear to be indifferent to Holly, though Leechy has expressed interest in seeing how Holly developes, likely due to a scientific interest more than anything else. Professor Oxblood treats Holly like the rest of the students, though a brief bout of hostility when Holly "interrupts" his class after Stoker's prank suggests he might have a personal opinion about Holly being half-mortal that he is keeping to himself. He gives her one hour of morning detention. Oxblood later gives Holly's assigned detention to Stoker when he learns about Stoker's prank and apologizes to Holly.