Green and Diantha’s friendship/relationship is actually really cute and sweet?
Look how much smaller she is than him in this flashback. She looks to be in her preteens/early teens while Green is likely 18-19 here.
Diantha and Gurkinn both (fondly) remember Green as being a bit of a d*ck, but he still apparently told a young girl epic stories about how some kids took down the criminals terrorizing his home region. He fed into her hopes and dreams, and then she eventually became the Champion of her own region.
So yeah, Green confirmed as best big brother.