Setsuki would either want to be right up in Valeries face, doing grabs, A,A or flipp mixups.
Or she wants to be outside of Valerie f+A. While Valerie wants to be outside of Setsukis A attack, but reach with her own f+A.
In the air, Valerie has a higher reward for beating Setsuki out of anything, since air A,A gives a ground bounce, and then that is 3 hp taken away from Setsuki. At that point, Setsuki needs to be aware of chip from the air super from Valerie, if she is within range. Setsuki is unable to convert any air to air attack, into real damage. But the air-grab-flip move she has, leads to good KD.
In the end, I am not too knowledgable about the matchup myself, but I would imagine that Setsuki wants to set up KD situations with hard-to-blockables, that lead into new KD situations.