This topic will probably go unnoticed, but I don't care. She's a boss.

Best known as voice actress of Tsurumi in Ano Hi Mita Hana no Namae wo Bokutachi wa Mada Shiranai
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and Sachi in Sword Art Online.

This song might be familiar to some of you; SCANDAL covered it too!
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After you take a listen to the seiyuus' cover (which I think is better than SCANDAL's or even the original...don't kill me please) of ZONE's song, you'll notice a very particular, emotional voice that stands out (starts at 0:57). Yep, that's Saori's. So I went and look her up and found more awesomeness:

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I'd list more but this song just underlines her great, great singing. Man, such a powerful, emotional voice. Hard to find that nowadays anywhere, let alone in Japanese music. She should seriously consider a singing career after seiyuu work, I'd buy all her songs...

@kaitanuva

I noticed your topic and I agree with you, she is amazing [You must be registered and logged in to see this image.]

Hmm, to be honest, I had expected more of that very particular emotional or powerful voice you described here, kaitanuva. I think Saori Hayami has a somewhat pleasant and subtle warm voice, but it did not came to my mind to describe hers as powerful. That last song has an apparent relaxed mood, and Saori adapts to that very much, especially noticeable in the way she ends her sentences. I think that songs does not really displays her vocals skills that way, because it sounds so... modest? I mean, I don't want to sound too hard, but when I think about singers who sounds powerful, I expect someone who is definitely skilled with breath-control and the like. But I cannot sense that - let it be clear - in this one song.

And well, there are a lot of singers who have really emotional and powerful voices, but it is harder to find them for they are mostly not the most popular... But I do not know if I would include Saori among them at this moment.

I hope you can list more songs, I will listen to them too! =)
I am aware judging with only one example can be really misleading...

I tend to exaggerate when I am in the process of discovering an artist I like. Upon further listening, I agree her voice is not "powerful", but "skilled" and sharp. I still love her voice though.

Here is another song I am falling in love with from her:

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She does shows some power singing here.