For sure Harry is there. But there could be new characters introduced. Maybe a foreign wizard who has come to Hogwarts for the summer and joins Harry in his training. The Weasley family could also be involved. Mrs. Weasley might be sending care packages to Harry during his training. And Mr. Weasley could be sharing some of his knowledge about the Ministry of Magic's regulations regarding certain types of training that Harry is doing.
Maybe Harry could meet new friends while training. He might discover new magical abilities related to his training. Or he could face some challenges that test his limits.
Obviously, Harry himself would be involved. And of course, there would likely be healers at St. Mungo's Hospital for Magical Maladies and Injuries. Hermione might also be there to support Harry, using her vast knowledge to help understand any complex medical conditions or procedures.
The teachers, such as Professor McGonagall or Professor Snape, would most likely be involved. McGonagall could be the one administering the test, while Snape might be observing with his usual critical eye. Also, other students in his year could be taking the test alongside Harry. They could be either competing with him or collaborating in some parts of the test. This would add an element of drama and friendship or rivalry to the fanfic.
Snape could also be part of it. He might catch Harry in a rather awkward moment when he's trying to hold it in and give him a hard time. Or, on the other hand, he could surprisingly offer some help in a very Snape - like way.
Dumbledore could be involved too. He often has a deeper understanding of things in the wizarding world. He might be the one who arranges the heritage test for Harry, or at least be aware of it and offer his wisdom and guidance. Also, some of Harry's professors could be involved. For example, Professor McGonagall might be curious about the results and how it could affect Harry's performance in school. And, there could be some characters from Harry's family's past that make an appearance, either as memories or some sort of spectral form during the test.
He could meet other young wizards and witches who are also working summer jobs. Maybe some from other wizarding schools.
I don't know who the author of this fanfic is. There are so many fanfic writers out there, and without more information, it's impossible to determine.
His friends might be involved in some way. They could be trying to protect him but something goes wrong and Harry ends up passing out. For instance, they might be using a spell to shield him but it backfires.
Sirius Black could also be part of it. He was Harry's godfather. He might have escaped from Azkaban earlier in this fanfic. He teams up with the Potters to rescue Harry. He could be the one who scouts out the Dursleys' house before the actual rescue operation, using his skills as an Animagus to stay hidden.
It could be Sirius Black. He has a lot of experience in the wizarding world and, despite being on the run, he could find a way to communicate with Harry secretly. He could teach Harry about dark magic defense, which would be very useful in the tournament.