Moto Sentouyou Dorei desu ga, Tasukete Kureta Ryuujin wa Tsugai Dasou desu

Eight has been forced to fight in an arena as a slave day by day for as long as she can remember. One day, however, all of that changed when a garrison of troops staged a raid on the arena, led by a Dragonkin named Celeste. In the frantic fighting and fleeing, Eight is seen fighting against her master... and in that moment Celeste realizes that he has found his destined partner... his mate.

(Source: MangaPlaza)