Story: I have to give this series a solid 8 for its story, it was satisfactory. Originally I thought that it would be hard to make a good story/setting primarily focusing on a platonic relationship between a brother and sister, but I was certainly proven wrong. The bonding between Choco and Kawagoe will take you on an emotional ride that will make you laugh, smile and quite possibly jerk a tear(it did for me). Only thing I can really criticize about the story is there isn't enough back-story on the main characters.
Art: For how much I loved the story, I certainly can't vouch for the animation. Now I'm not saying the animation is terrible in Chokotto Sister, but it definitely could use some improvements. Though usually the fanservice shots tend to have better quality lol.
Sound: To be honest, most people could care less about the sound effects in a drama/comedy. In this genre, as long as the sound effects don't sound totally out of place, then I'm happy, and the sound effects in Chokotto Sister sounded just fine to me. As for the music, I'm pretty sure you will get tired of hearing a certain dancing song after a while but its nothing I would hold against the anime lol. The ambient music sounds just about right and I really liked the OP and ED.
Character:Though some of the characters in Chokotto Sister may come off as generic(tsundere, cute/dumb, etc.), the way they portrait the characters is decent enough. I never found myself asking "well why is that character even in this anime?" which I have definitely asked of other animes.
Enjoyment: I enjoyed Chokotto sister very much even though it does tend to have a little excessive fanservice(it's a partial oppai anime though so what do you expect?). I probably enjoyed it even more since I watched it in the Christmas season.
Overall: Overall this anime is an instant classic Christmas holiday story that will touch your heart despite its flaws. I probably would have given it higher score if it had better characters, after all, it is a character driven show.
