Miya twins during elementary school.
Background
Atsumu lives in Hyogo prefecture, and thus speaks in a Kansai dialect. The twins have been playing volleyball since their elementary school days.
They attented a volleyball workshop with another one of their teammates - Ojiro Aran. During the workshop, Atsumu witnessed personally how a skilled setter brought out the potential of their spikers by sending them the best tosses. This piqued his interest in becoming a setter.
At first, Osamu was picked as the setter due to being more skillful. However, according to Aran, Atsumu is said to have an irrepressible hunger as he pushes himself to improve.
Eventually, Atsumu overtook Osamu as the official setter of the team.