#basically siblings

They knew of each other due to being in the same year and classmates but, had never interacted before their assignment for Transfiguration in year 2. Ambrosius left Hermione to do all the work, a strategy Hermione had heard about when Neville Longbottom had mentioned how Ambrosius was when they got paired in Herbology but, Neville was more than happy to look after the plants himself.

Ambrosius managed to avoid Hermione for three days until she cornered him after breakfast one morning, telling him if he didn't meet her in the library after classes ended, she'd go straight to McGonagall. After that he met her everyday in the library after classes where they ended up learning a lot about each other due to Ambrosius asking Hermione about herself to avoid working on their assignment.

After that the two became friends, when the golden trio planned to sneak into the Slytherin common room Ambrosius got the hair samples for the polyjuice potion. After seeing the failed polyjuice failure on Hermione he stayed with her, trying to comforting her and apologizing for getting the wrong hair. He even helped her to the hospital wing by trying to keep her out of sight. Ambrosius went on to continue to help the trio with the chamber of secrets.

In year 3, Ambrosius tried to avoid Hermione and the rest of the trio as to not get dragged back into whatever mess was happening but, also because he was grieving the loss of his Mother. Hermione helped him through the grieving processes by lending her ear and shoulder to cry on. After year 3 Hermione and Ambrosius would start to meet up regularly during the summer. Since both had access to telephones they'd often speak through the phone rather than send letters.

They'd grow closer as they got older, Hermione would be the one to find him in the Forbidden Forest after being bitten and turned into a werewolf. (I'll make a post about Ambrosius lycanthropy eventually) Hermione would read through any and every book about lycanthropy she could get her hands on, She'd help with the aftermath the full moon with numbing potions she'd brew to ease the pain of the transformation, and spells to help muffle sounds and smells so they didn't overwhelm him anymore than he already was.