In Defense of Eirika (FE8 Ch. 18A)
The fandom is known to constantly shit on Eirika for giving Lyon the Stone of Renais in her route in Sacred Stones because they think it was a stupid, naive thing to do. I disagree. While it’s certainly bad writing and another example of how the narrative of FE8 treats Eirika like dirt in comparison to her brother, character-wise and even story-wise it does make sense.
In Chapter 17A Lyon approaches the twins, not acting like himself. Ephraim gives Eirika a short rundown of what he discovered while he was in Grado - that Lyon’s been manipulating them the whole time, etc. Lyon, or really, Formotiis, reveals that he’s been possessed by the Demon King. However, the regular Lyon does shine through at times during this cutscene, which leads Eirika to believe he can be saved.
This brings us to the Chapter 18A cutscene. Lyon tells Eirika in his brief moments of control that he can be saved, but only by use of the Sacred Stone. Eirika is a tad skeptical, but chooses to trust him, giving him the stone.
Now, let’s break this down, starting with why this fits character-wise. Eirika is shown to be very compassionate and cares deeply for her friends, especially Lyon. She’s also shown to take risks in order to help people - in Chapter 6, she’s willing to hand over her bracelet to Novala to save some citizens until Seth stops her. Not only this, but Lyon said he’d die if she didn’t give him the stone. Eirika’s choice was this: risk the stone to save Lyon (one of her best friends), or refuse and let him die. It’s very in character for Eirika to prefer to risk the stone and live with those consequences over risking letting Lyon die when she could have helped him.
It also makes sense when we consider two things: one is Magvel’s mythology, which says that the power of the Sacred Stones can purge the Demon King. Eirika knows this, having learned about it in school, and also knows that Lyon’s been able to perform a miracle using the stone before.
The second thing we need to look at is how Eirika finds out Lyon’s possessed. Unlike in Ephraim’s route, Lyon doesn’t reveal that he’s been manipulating the twins, instead playing the ‘I’m possessed against my will’ route. Ephraim does tell her smatters of it, but it happens right in Chapter 17, while Lyon is there. It’s a bit of a stretch, but we can’t guarantee what Ephraim said really registered with Eirika, and again, Lyon was playing up the possession side of things, so for all Eirika knew, that was just the Demon King spreading lies.
TLDR; based both on Eirika’s character and the circumstances of the scene, while it’s bad writing the fact that she gave Lyon the stone shouldn’t reflect so badly on her.