I don't know if you pronounce "missy" differently than I do, but I pronounced the second syllable "see".
And to me, Mandarin 'xi' sounds a lot more like "she" than "see".
And to me, Mandarin 'xi' sounds a lot more like "she" than "see".
On the other hand, I'm a Southerner, so I don't usually distinguish between "sh" and "s" sounds. I can do the 卷舌 thing, though, well enough that Northerners tell me I'm doing it right, and I don't think either Pinyin "shi" or Pinyin "si" sound anything like English "she". And while I don't think Pinyin "xi" sounds exactly like English "she", I think it sounds pretty close.