The house elf might be very grateful. Harry is known for his kindness, so the elf could see this as a chance for a better life. It might immediately start serving Harry with great enthusiasm, believing that Harry will be a fair and good master.
At first, the elf could be a bit suspicious. In the wizarding world, house - elves are used to a certain way of life, and being bought by Harry might seem strange. However, as Harry shows his good intentions, like giving the elf some freedom and treating it with respect, the elf could slowly warm up to him. It could start to open up and share its knowledge and skills with Harry, and maybe even develop a sense of loyalty that goes beyond the typical master - servant relationship.
Well, he might use the house elf to keep his new house in order. The elf could help with cleaning, cooking, and protecting the place from unwanted intruders. It would be interesting to see how Harry, who has always been against the unfair treatment of house - elves, would manage this new relationship with an elf he owns.
Well, perhaps the house - elf is constantly tidying up in really cool ways. It could be folding clothes by magic so that they are always perfectly creased, or it might arrange the books in Harry's library in a way that makes it easier for him to find the exact spell book he needs at any given time. And it could also have a special connection with the other magical creatures around the house, like communicating with the garden gnomes and getting them to stay in line without causing trouble.
There could be many possibilities. It might be an original character created specifically for the fanfiction. However, it could also be a well - known house - elf from the original series, like Dobby, who in the fanfiction somehow comes to be owned by Harry Potter in a different set of circumstances. Maybe Harry saves Dobby in a more complex way in this fanfiction and as a result, Dobby becomes his house - elf.
Perhaps it could start with a charismatic leader among the house - elves. One elf with a strong vision could inspire others to join. They might gather in secret at first, training in the hidden corners of Hogwarts or other wizarding estates.
A house - elf can be significant by being a key informant. Since they are often overlooked, they can overhear important conversations. For instance, they might overhear Voldemort's minions planning something in a wizard's home they serve. Also, they can use their unique magic, like their apparition - like ability to get in and out of places easily, to help the heroes. They could transport secret messages or objects.
Dobby is often very popular in fanfiction. Since he was already a rather unique and brave house - elf in the original series, in fanfiction he can be portrayed in even more heroic or complex ways, like leading a revolution among house - elves.
Dobby would be a very likely candidate. He was already a prominent house elf in the original series and had a strong connection with Harry Potter. His brave and rebellious nature would make for great stories in a fanfiction where Harry is involved as a house elf too. Maybe they could have more adventures together, this time with Harry sharing Dobby's experiences.
Dobby is definitely one of the most popular. His character from the original books was so endearing and his actions so brave that he often features prominently in fanfiction. Another popular one is Kreacher. His complex relationship with the Black family and his growth as a character make him a great subject for fanfiction writers.
His personality would be significantly different. Since house - elves have a very different culture from wizards, Harry would grow up with different values. He could be more empathetic towards the underprivileged in the wizarding world, like the house - elves themselves. He might also be less concerned with the pure - blood/muggle - born divide, as house - elves don't really care about such things. Additionally, he would have a strong sense of duty towards those he considers family, which in this case would be the house - elf that raised him. This sense of family and loyalty would be a core part of his personality.
In fanfiction, a Harry Potter house - elf army could be very powerful. They are known for their loyalty and magical abilities, especially in domestic magic. They could use their skills to protect Hogwarts or support a particular side in a wizarding conflict. For example, they might use their ability to apparate within the castle to quickly move around and surprise enemies.