Something I think I'd heard once, but I'm not 100% sure on, is that "Esther" was the official spelling, but in scripts it was written as "Ester" to ensure the voice actors pronounced it properly. So either way could be considered correct.

Violet'll likely know more about this than I do.

I've heard the theory above, too. And while I may not do the answer as much justice as Vi, I thought I'd pitch in my two cents... 

I seem to recall that, back when the Samurai Pizza Cats were broadcast, the Closed Captioning option read Polly's name as "Polly Esther." However, I also believe (And on this I may be mistaken; I'm positive on the CC 'Esther') that the European re-packaging of Japanese KNT toys read, "Polly Ester." 

In the end, it probably just boils down to personal preference!

Hi, all! I'm new, but I'd like to address...

Emperor Fred is too scatterbrained for anyone to be with, His family only understand and tolerate him.

Oh, but what about Usa no Tsubone/the Empress Freda (Alternately spelled Freida, Frieda, etc.), otherwise known as "Princess Vi's Hippy Dippy Mom"? I always thought she and the Emperor were very much in love - they had Vi together after all! Besides, in that one episode, she gives Emperor Fred the most enthusiastic greeting anyone has received in the whole series, in my opinion.  Though, only a few seconds after whirling him around gleefully, the Empress claims the only thing she sees in him is his wealth.

Still, they were always one of my favorite couples in the series. Perhaps the reason for their blissful marriage is because she travels the world so much; she doesn't have to put up with Fred on a day-to-day basis!