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

S-Rank adventurer and second-generation sword saint, Ryunoa Styler, is reunited with his childhood friend, Aine, who has been reduced to slavery. Ryunoa purchases Aine in order to save her, however, she's been given a cursed collar that causes the wearer to go into "heat" once a day...