Asriel!

Now hear me out:

Fiona Kid is never really mad((If she was Asriel, she would as well be Flowey)), only happy and depressed((she lost her parents and has still not found her sisters. Why wouldn't she?)). And because of that, she would never be enraged. Just plain sad. But she has happy moments. Like when Lilliby gives her cake or when she hangs out with her adult friends ((Before Sprisk came, she had no kid friends. No one at her school really liked her.)). 

And that's why I would mostly think Flowey/Asriel would be a perfect role for her.

I never really understood why people swapped MK with Gaster

Or why people would swap Flowey with Temmie

Okay, so I'll draw her as Flowey later.
Thanks for understanding!

-Fiona