It wasn't regret or pain. Gin was quite satisfied with his life, he couldn't claim it is the best amongst the other members of the team guardian but even then it was enough for him. He got to fight in the frontlines with Donald just like he has always wished to. He met great friends, he experienced his fair share of breakups and makeups. He fought against the death mage and also fought alongside the death mage countless times. In his opinion, many people will literarily beg to have the kind of life he lived and die a warrior's death like the one he was about to receive.
"Sorry… I am against killing female foes but I have a duty to accomplish as a member of the greatest adventurer team in the history of the kingdom of Gyro" Gin's eyes were void of any form of pain or sadness just determination.
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Zoom!