Well, Riza's probably not a common name in Japan, if I had to take a guess. (Oddly enough, there's all sorts of characters with the name "Risa" with an "s", but we don't talk about them here.

) And really, there are so many derivations of "Theresa" (the name meaning "harvest", as
Bett Bett mentioned). I mean, FMA was the first place I'd seen "Riza" with an "i", let alone anything pronounced as such without the "te" sound in front of it. No joke.
I did do some looking, though, and, as
Bett Bett is from Hungary, I'm not surprised at what I found: the almighty Wikipedia (Yes, yes, well all know how reliable it is, but I think we can trust it on this one) has "Riza" in that exact spelling listed as a Hungarian variant on the name "Teresa".
I will, however, also refer you to
Turdaewen's
post for another take on the meaning behind "Riza". Not saying it's necessarily right or that
Bett Bett is necessarily wrong, but just pointing out that there can be more than one take on a name. Interestingly enough, the name
Turdaewen mentions isn't among the variants Wikipedia gives. Hmmm...
Merely food for thought, as I said. I just don't know why Arakawa would knowingly choose a name meaning "harvest" for Riza, even if she just liked the sound of it or something, but not everything has to have a meaning behind it, I suppose. Ed and Al, and Roy's names seem to match just a little bit better, though, even if you take the old Celtic/Gaelic meaning of "Roy": red. (Referring to hair, but red=blood...?)
Okay, yes, I've thought WAY too much about this, but that's how I am. (That, and one of the Royai 100 themes is "Given Name", so I did a little bit of research when I wrote it.)
OT: Happy birthday,
Bett Bett! I hope the day has been/is/continues to be great!