>>530011780
Her cost reduction is quite simple, the copied skill card has its base cost reduced by 1, it's not a cost reduction buff in itself, so it stacks with cost lowering skills.
So if you use Rio on Iori, for example, the Rio'd EX skill will cost 2 instead of 3, if you have a cost reduction skill like Seia's active then she will cost 1 as (3-1) / 2 = 1.
Similarly if you use Mika, then the Rio'd EX skill will cost 5 instead of 6, if you have a cost reduction skill like Seia's active then she will cost 3 as (6-1) / 2 = 2.5 = 3