Seiko-chan (惺子), 18 years old, high-school student. (name roughly means "sincere child")
She's knowledgeable, kind, honest, helping and straightforward. And good-looking, of course.
Seiko-chan represents the greatness of StackExchange community, and, in their turn, users are inspired to become great like her.
She loves experiencing various emotions the world of anime and manga can offer through adventures, dramas, hentai, romance, comedy, sci-fi, slice of life, sports, fantasy and whatnot.
Seiko-chan's favorite drink is tea. Different kinds of tea, which perfectly fit the mood and allow for best entertainment experience.
Anime and manga just happen to be one of her main interests in entertainment. And so this is for many users, so don't forget to join the Chat for some on- and off-topic discussion!
[end of cool part]
Forgive me if I messed up with the Japanese, for I am lvl 1 in it.
If anything, I'd go with Shaft's art style for realistic proportions, attractive body figure and serious, yet adventurous feeling. Along the lines of Hanekawa Tsubasa of Monogatari series.
I imagine it would be pretty awesome to have a random rotation of header backgrounds of Seiko-chan with various cups of tea, reading some manga or watching anime, if anyone is up to the task of drawing them.
I'm opposed to cliche mascot characters with standard trademark elements, and StackExchange deserves much better. Which is why I thought of the above. Feel free to be inspired by this thought, and create your own unique proposition worthy of this site.