The forgiveness has to do a lot with his own self conviction. He originally went there to save her not just a child his understanding of her circumstances and that to him he wants to be not just a hero, but somebody that can make a peaceful world. Kind of you know the cool conversation that they were having in the first place is about character is about what you wanna do what you wanna be what you wanna follow what your standards are. Can you live up to them Part of that was literally that.
I think it was important in developing Shirone’s character and philosophy. From what we’ve seen of him he’s had this black and white thinking of bad and good( like Marcia pointed out to him). And him forgiving her shows the change in his reasoning, that he can’t judge others based on their actions alone— he needs to consider context. I also don’t think it was so much “power of friendship” or “this isn’t you” bs that pulled her out from her trauma, but Shirone’s sheer conviction and godlike selflessness in making a better world for all like Erys pointed out. I also think it would out of character for Shirone to kill Marcia when she was at such a fragile state because his flaw and virtue is his kindness.
It’s OK I get it when you bench something you kind of get into this lake because most of us read a lot of of this type so you just get into this like oh I know that I don’t like this because I read this too much. I know exactly what I like and I know exactly why I think this should go and I wanted to go so when I kind of deviates and you’re like oh I knew it. I hate this. This is gonna be so boring you just gonna get stuck into one direction and it sucks.

When Shirone pulls up that "Friendship card" or like that one scene in kissing booth "This isn't you" card I laughed and I still don't see the connection of Marcia to Kergo or Miro like bruh why did they forgive too quickly? Kind of messy but let's see how this one goes.