D: When Koala fell around Robin's neck , I came to like her \(^ ͜ ^)/. So exactly under what circumstances did Koala, Hack, and Sabo meet?
P.N. One Piece Mania
O: Hm, many people want to know about the life of these three. Once I start to tell you there'll be no end.
Firstly, there is Sabo who was rescued by Dragon when he was around 10 years old. He then meets the Revolutionary Army and learns about things happening in the world, and their (Revolutionary Army) thoughts on it. Finally, he sympathizes with them.
That's when he meets Hack who, as a master in Fish-Man Karate, teaches martial arts to children without prospects that are raised by the army. However, Sabo carries no memories. But he isn't a boy anymore on a level such of someone who can still be taught something. Taken under Dragon's wing he shows great skills and grows rapidly.
That's when Sabo is 13 years old. A 14 year old girl called Koala is brought along by Hack and joins the Revolutionary Army. (Skipping some details) She takes Fish-Man Karate lessons under Hack and even becomes Master Assistant. The obliging Hack and Koala are, together with Sabo, increasingly assigned to missions as a team due to their good affinity.
Yes, the order of their encounter has this kind of vibe. The story is broached in the anime, but I think there is not enough room to tell everything. Please figure it yourself.