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

Story:

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… Ryunoa's battle to find a way to remove the collar starts now!