I think it's either he realized she's important to him before leaving or in Arensbach and both times he can't even consider her, because she's a child. But when she grew up, he was happy that there are parts of her that remains the same. (and she's now in his strike-zone and therefore, he has plans and you know what happens when ferdi plans ) She's very important to him, enough to be zent(ruler, very stressful) if she wants him to. i think, to him, she's like warmth when you've lived in a very cold place for a very long time, like he's being thawed and he doesn't want to be frozen again. (sorry not sorry for frozen ref haha) Their romance is very funny because myne is very dense and clueless and ferdi is a noob with feelings. There were scenes that were very ecchi in his perspective but to myne, it was just sweet (she's a bit stuck being childish) and she doesn't undersrand that she's provoking/seducing him unintentionally and she's a minor(it's a mix of being very mature and very innocent as she has her previous life's memories). So ferdi will probably attack her or give in to her provocation when she's of legal age.