[Illsteir]

Originally Posted by Kanon

The answer to that is on the official website (the Japanese one).

Spoiler for Just in case people don't want to know:

The mother specifically sent Suletta to that school as part of a ploy to take revenge on the people who killed her husband and colleagues. Suletta doesn't know, the mother only told Aerial who didn't want Suletta to get involved because she's a very sweet girl. Sadly, Mommy can't hear Aerial's voice, and her mind is set on revenge anyway.

Suletta becoming Miorine's fiance is part of the plan by the way.

Man, I'm really a sucker for this kind of trope in mecha anime; the machine being a total bro (or sis in this case) to its pilot.
And as someone pointed out in the YT comments, the lyrics of the OP is a POV of Aerial to Suletta.