Using the blue to white background with the dark edges really works well with this piece. Overall you drew her so cute and adorable. It is a new look on Sayaka. Most artists draw her last moments and add music notes but this piece shows the happier times. The cape and outfit have to be the best parts of this unique piece. The cape matches the outlines by giving that 'watercolor' blending technique but you can tell it is different from everything else. Also the outfit matches what she wore in the shows.
Some bad points of this piece would be the sword and the lips. The sword is very basic and follows a simplistic look though looking at other comments I see your not used to drawing swords so for a novice I give you good points for it. The lips at a glance looks like she is sticking her tongue out at the viewer and gives a different feel to this piece but upon closer look it is known that she isn't.
Overall this piece is cute and has a very unique art style that I don't see very often. I love the boots, and outfit and hair.