Mafalda Hopkirk was a witch who worked for the Ministry of Magic. She was an assistant in the Improper Use of Magic Office.
This is an interesting question because while Mafalda didn’t physically appear in the story until the Deathly Hallows, we first came across her in letter form – when she wrote to Harry in Chamber of Secrets. It wasn’t the most pleasant of correspondence. She told him that if he performed magic again outside of school, he would find himself expelled from Hogwarts. Rather unfairly, it wasn’t even Harry who was responsible for the Hover Charm at Privet Drive – it was Dobby the house-elf.
When we first met Mafalda in person, she was on her way to work at the Ministry of Magic. She had just Apparated into a tiny alleyway – so as to not be seen by Muggles – when she was hit by a Stunning Spell (cast by Hermione Granger) and promptly dragged into a deserted theatre out of view.
We don’t know an awful lot about Mafalda’s appearance as we only saw her briefly before she was Stunned. Though what we do know is that she was small, had flyaway grey hair and wore glasses – which Hermione took from her and wore while impersonating her using Polyjuice Potion.
There are a couple of reasons why Mafalda Hopkirk mattered. Firstly, she was the one who wrote to Harry when he (or Dobby) performed illegal underage magic outside of school. Not only did she send Harry a warning in Chamber of Secrets, but she was the person who told Harry that he had been expelled in the Order of the Phoenix (before letting him know that he had to attend a Ministry hearing instead). Her letters reinforced just how serious the Ministry of Magic was when it came to this rule.
However, it was in Deathly Hallows that she played her most important role… or at least her likeness did. When Harry, Ron and Hermione decided to break into the Ministry of Magic – to retrieve the locket Horcrux from Umbridge – they decided the best way to get in would be to disguise themselves as three Ministry employees. Mafalda Hopkirk was the witch that Hermione decided to masquerade as using Polyjuice Potion. Camouflaged as the Improper Use of Magic assistant, Hermione managed to get close to Umbridge and sat in as a notetaker for the Muggle-born Registration Commission. There she helped to free Mary Cattermole and other Muggle-borns who were destined for unfair hearings, the destruction of their wands and worse.
Plus, once Harry had Stunned Umbridge and taken the Horcrux, Hermione cast Geminio to duplicate the locket so that Umbridge wouldn’t realise the real one was missing when she came to. Her and Harry were then able to work together to escape. If she had been disguised as any other Ministry employee their mission may not have been successful. And if they had failed to retrieve the locket, they would never have been able to defeat Lord Voldemort.
We can understand why Mafalda might have slipped your mind. Her name was only mentioned on ten pages of an entire book series that had many different characters. And for a lot of that she was only mentioned in letter form. In fact, she only physically appeared as herself once – anything else was Hermione posing as her. That being said, we think it’s well worth remembering this Ministry witch. We believe that she inadvertently helped to bring down Voldemort… thanks to a few hairs and a well-brewed Polyjuice Potion.