Pretty sure this is supposed to be Reimu and Marisa's child. I'll remove the cosplay tag and add it to the "Let's Make Touhou Kids" pool. Feel free to correct me if I'm wrong.
I like Aniki just seems easier to say but the closeness conveyed with Big Brother seems more appropriate for this peace
On the other hand, she first uses it in the first koma where she's being more tsun. I like aniki more overall, though big brother would be fitting in the second koma.
So that's who she was. I kept searching for Valkyrie-related cards in YGO wikia, but I guess this is one of those cards where they threw the original name out the window.