I don't know tho because even though he said it was only a little he said if she drank even a little it could've been fatal so I think it's a strong poison that would've killed her but didn't have an effect on him only because he's been building immunity for so long
That’s definitely not it. The Duke wants to keep her by his side not kill her. Her body isn’t trained to handle poison like the duke and Ein can. Ein said even a small amount of poison can be fatal and possibly kill her. Also the duke and Ein is keeping a lot of sercets from her so it’s makes no sense for them to start to train her now when all they want to do is basically keep her in a bubble by their side forever. It’s is a good theory tho just very unlikely of it being true.
maybe the duke is behind it.feeding her poison little by little so that she will be immune for future accident.. who knows..ein said its only lil.