There is a line of Blade Quests revolving around a pair of sister Blades, Praxis and Theory, who are stealing Core Crystals for a group of criminals, often killing people to do it. You spend a while, with aid from Mor Ardain's military, preparing to set them up for an ambush in two separate locations. Praxis and Theory arrive in one location along with Theory's Driver, but Praxis' Driver arrives in the other location. Praxis' Driver gets killed by the Mor Ardain military off-screen which results in Praxis returning to her core crystal. When a Blade returns to their core crystal, they lose all memory of their previous life. Theory and her Driver flee the scene, while Rex's gang takes Praxis' core crystal. You can resonate with it after a bit of time.
After resonating with Praxis, you can start a Blade Quest where Theory spies on your group and is trying get Praxis to remember her past, while Rex's gang keeps that info from her. After it's made clear who is sending Praxis the letters, Rex decides to tell Praxis about her life of crime and bloodshed. Praxis can't remember any of it due to the way Blades lose their memories when returning to their core crystal. However, she wants to connect with Theory again as she feels an innate sense of familiarity with her. You then confront Theory and her Driver and eventually, Theory kills her own Driver so she can be with Praxis again.
Also, a lesser example would be Perceval, a Dark type Blade. In his previous life, he was a ruthless assassin, which you discover in his Blade Quest, apparently having even killed innocent people, including children, to accomplish his goals. However, he doesn't remember that when Rex and co. picks him up (he's one of the Blades you can pull randomly), and in his current life, is actually a pacifist who refuses to kill people and Blades, but is still passionate in pursuing justice.