The interview goes over several points; I will highlight some in quotes.
ND: I see. But it must be hectic to choose characters based on their relationships with each other. You might even end up having too many characters that couldn’t be removed. For example if you were to add Chrom, then how about Lucina?
Hayashi: But actually, we used to stray away regarding Lucina.
ND: And what do you mean by that?
Hayashi: At first there were talks in the development team of, “Should we remove Lucina?” However, after we talked with Nintendo and Intelligent Systems, we reconsidered that, thinking “That’s just not possible after all” (laughs).
ND: Why did you want to remove her?
Hayashi: It’s an obvious thing; it’s not because we hated her or something, but the reasons are because there are already too many sword characters, and because she’s of the children generation.
Usuda: Which means, when we were contemplating on whether to add people from the children generation, if Lucina wasn’t added, then we would’ve come to a conclusion that we wouldn’t add anyone from the children generation.
So Lucina was originally on the chopping block, which may shock quite a number of fans. However later on they stated that characters that shared the same class became more of a concern due to Lucina’s inclusion.