Nakashima Reiko is Yoshi's nemesis of sorts, who used to believe in the power of Redemption, but became Bitter and Jaded due to an incident in her and Yoshi's backstory. She was hunting The Shadow, the mastermind behind whatever evil plot it was that got her family killed. The party still doesn't know what the evil plot was.
Her philosophy is Aku Soku Zan - to Slay Evil Immediately. Though she takes the time to torture information out of some of her victims, first.
Per her philosophy, Reiko's fighting style focuses on speed and lethal first strikes.
According to the party's best information, she hasn't killed any innocent people yet.
Yoshi asked the Great Dog Spirit to send her a dog, hoping it would be a reminder of unconditional love and a companion to keep her from being alone with her anger. She named the dog "Faith."
She became The Shadow to save the lives of Yoshi and his friends in Chapter 3.
One day Reiko and Yoshi came upon a pair of bandits accosting a traveler, and quickly put the bandits at their mercy. The terrified bandits begged for mercy, and Yoshi convinced Reiko to allow them to live. In retrospect, Yoshi should have recognized that their fear was not entirely directed towards the warriors in front of them, should have realized these were no ordinary bandits, should have realized that they feared failing their leader and realized what that meant. But he didn't, and he knows and admits that it was because he was too eager to prove his merciful nature.
Hungry for vengeance, the bandits tracked down Reiko's family. Reiko's family were not warriors, and the bandits slaughtered them and made sure she and everyone else knew who had done it, as a warning not to defy the bandits. Reiko was furious - at Yoshi as much as the bandits. She rebuked his attempts to reach out to her and set out on a bloody path of vengeance, stooping to gruesome methods to track down and destroy every last one of the bandits. She sees her goal as justice, but she is ruthless and merciless in carrying it out. She aspires to impose her personal justice on the world, but part of her must feel the pain of causing so much bloodshed. If Yoshi could achieve the same result without that stain on his soul, it would be unbearable for her. That is why she felt the need to convince Yoshi of the rightness of her philosophy as much as he longs to persuade her to abandon it.
(these are notes on Reiko from before she appeared in game - actual Reiko may vary)
Philosophy: People are inherently evil and selfish, people in power got there through ruthlessness and are typically corrupt, non-evil people should be protected (and taught to defend themselves), life is cheap, people are persuaded only by fear or by steel
Opinion of Yoshi: he's a rare good person - he should be helping me but he refuses to do what has to be done and it got people I care about killed - when I show him the necessity/success of revolution/executions/fear he'll join me - (subconscious: killing/terrorizing people weighs on my soul and I can't accept his philosophy because it means I didn't have to cause all of this carnage in order to protect people))
She thinks that evil should be cut out immediately because people are easily corrupted. "Aku Soku Zan" (destroy evil immediately) sums up her mindset. No patience, no forgiveness. The enemy is too dangerous to give it a second chance. She may have pre-emptive killing schticks or other interrupts (think iaijutsu - the art of drawing the blade and striking in one motion - an iaijutsu master would seek distance in a combat to resheath the sword for another bout - kind of like jousting. You sacrifice defense to put yourself in a position to launch a stronger attack). I think "people are inherently evil" may be too strong a sentiment for her, but she definitely sees evil as very scary and very powerful and very likely to get a hold on people. In terms of ruthlessness, she will kill enemies or traitors without a second thought, possibly in a terrifyingly brutal or grisly way, but she is not so much into throwing away the lives of those under her command. She is less of a good judge of character than Yoshi, and her reaction to this is to remain on-guard and suspicious with those outside of whatever she defines as her group.