This is one of my favorite character turns in all of 'Bungo Stray Dogs'. Chuuya at fifteen is basically a contained explosion, and the arc defining him is the 'Dragon's Head Conflict' prequel light novel, 'Dazai, Chuuya, Age Fifteen'. The key event is, without a doubt, his initial partnership and subsequent 'defeat' by Dazai. He's introduced as the terrifyingly powerful leader of the Sheep, a street kid defending his turf, only to have Dazai outmaneuver him completely not through power, but through cold, flawless strategy.
That first loss fundamentally reshapes Chuuya's world. He's forced to join the Port Mafia, the enemy, because Dazai proves the Sheep's loyalty was conditional and fragile. The real gut-punch is the 'Assassination King' incident, where Chuuya thinks he's finally getting a win by taking down Randou, only for Dazai to reveal it was all a setup to test his loyalty and resolve. The look on Chuuya's face when he realizes he's been played again is brutal. It cements their dynamic: Dazai the manipulative genius, Chuuya the raw, powerful force being sculpted, however painfully.
And then there's the confrontation with 'Arahabaki'. The revelation that he might not even be human, that his overwhelming power is a curse he's been carrying, and his defiant choice to use that power anyway to protect Yokohama alongside Dazai against the dragon. That's the birth of 'Double Black', forged in mutual distrust and a shared city. Those events at fifteen turn the feral, prideful boy into the disciplined, fiercely loyal executive we meet later, but you can always see the cracks from that year.