I like the concept of the story: How the girl accidentally happens on MissingNo., who calls itself "an error in the code", and now I'm imagining the trainer finding out the truth that she was created in a game. Also, I love how she tries to use politeness by calling MissingNo. 'sir'.
Was the code supposed to mean something in binary? How can MissingNo. say what it does - Like "pm ;smd" and "qgR;A" and the random capital letters? Does what it say appear in front of the trainer? And "Sylph" is supposed to be spelled as "Silph".
All in all, a really good oneshot. Loved the whole flow and the words used. Haven't seen a lot of MissingNo. in Pokemon fics.