Maybe Neville. He's a kind and understanding friend. He could encourage Harry to be brave and face Parvati to apologize. Neville could also share his own experiences of making mistakes and having to apologize, which would give Harry more confidence to go through with it.
Hermione might help Harry. She's always good at giving advice on how to handle social situations. She could tell Harry exactly what to say to Parvati to make his apology sincere and effective.
Luna could be the one to assist Harry. Since she has a unique perspective on things, she might give Harry some off - beat but very useful advice. For example, she could suggest Harry give Parvati a special charmed object as part of his apology, something that represents their shared Hogwarts experiences.
Hermione would be a likely helper. She's smart and practical, and she could help Harry figure out what to do with all the stuff in the vaults, like how to properly inventory and distribute the items.
His friends, Hermione and Ron, would surely help. Hermione with her intelligence and Ron with his loyalty.
Neville could be another helper. He has become quite skilled in herbology. He could help Harry by creating a magical garden around the house. The plants in the garden could be used for protection, like thorny bushes that keep unwanted visitors away, or for providing ingredients for household potions.
Dumbledore might help from beyond the grave, in a sense. Harry could discover some of Dumbledore's old notes or memories that provide crucial insights into building a powerful and controlled bomb. And perhaps some of the more eccentric professors at Hogwarts, like Professor Flitwick, could offer their expertise in charms which might be needed for the bomb's construction.
His friends like Ron and Hermione would surely be there to help. Ron could assist with things related to food and having a good time, like trying out different wizarding treats while Hermione would be great for helping with research - like finding out more about his parents or learning about new spells.
There could be a group of magical outcasts in Canada who take Harry in. These are people who have been shunned by the mainstream wizarding society for various reasons. They would understand Harry's need to escape and would protect him because they see him as one of their own, someone who is also trying to find a place where he can be safe and free.
Hermione would be a likely helper. She's always full of knowledge. She could research about simulacrums in the library and find some old spells or precautions that Harry should take while creating or handling it.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
Dumbledore could play a role too. He has vast knowledge and connections. He might give Harry some hints or direct him to certain places or people who can assist in finding the cure for lycanthropy.
Since it has 'Sakura' in the title, Sakura is likely to be one of the main characters. There could also be other characters who are related to the reason for the apology or those who are affected by it. Maybe there's a character who is on the receiving end of the apology.