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.
House - elves can be significant in fanfiction through their loyalty. If a house - elf befriends a main character like Harry, they will do anything to protect him. Their loyalty can be a driving force in the story. For example, they could sacrifice themselves to save Harry from a dangerous spell. Moreover, their connection to the wizarding households gives them access to hidden secrets and passages. They can guide the characters through these unknown areas, leading to new adventures and discoveries.
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.
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.
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.
If Harry becomes a house - elf, his character would develop in a way that he becomes more resourceful within the constraints of a house - elf's life. He would have to use his wits to get things done because house - elves have limited magic compared to wizards. Also, his sense of justice would be even stronger. He would be constantly looking for ways to break the chains of the traditional house - elf servitude, maybe by finding loopholes in the ancient laws that bind them.
Dobby's character could be developed further by showing his growth in confidence. After being freed, he could become more assertive in his actions and decisions, not just relying on his loyalty to Harry but also on his own sense of right and wrong.
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.
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.
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.
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.