My childhood friend who I used to train swordsmanship with became a slave, so I, as an S-Rank adventurer decided to buy her and protect her.

Alternative

Issho ni Ken no Shugyou wo Shita Osananajimi ga Dorei ni Natteita no de, S-Rank Boukensha no Boku wa Kanojo wo Katte Mamoru Koto ni Shita My Childhood Friend Who I Used to Train Swordsmanship With Became a $l@v3, so I, as an S-Rank Adventurer, Decided to Buy and Protect Her My Childhood Friend Who I Used to Train Swordsmanship With Became a Slave, so I, as an S-Rank Adventurer, Decided to Buy and Protect Her; 一緒に剣の修行をした幼馴染が奴隷になっていたので、Sランク冒険者の僕は彼女を買って守ることにした; Issho ni Ken no Shugyou wo Shita Osananajimi ga Dorei ni Natteita no de, S-Rank Boukensha no Boku wa Kanojo wo Katte Mamoru Koto ni Shita; Issho ni Ken no Shugyou o Shita Osananajimi ga Dorei ni Natteita no de, S-Rank Boukensha no Boku wa Kanojo o Katte Mamoru Koto ni Shita