Nintendo didn't twist his arm. Sakurai does has complete control over the roster. He was hesitant, but no one pushed him. Basically he said, "hey I want to include this guy, but I'm afraid of too many FE characters." Then his staff said, "Nah, he looks fun. People won't care" And Sakurai said, "k".
Underline done by me.
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Source
It also doesn't change the fact that he didn't
have to include a character for promotional reasons. He chose to make that "slot" and then in the end he chose Corrin to fill it. Like I stated in my
Corrin Controversy article, I do feel that Sakurai boxed himself in with this decision and neglected the overall DLC picture.
If you are a fan of Corrin's inclusion then that's cool. I just think having ~30% of all DLC characters to be Fire Emblem is a bit unfair to non-FE fans.