One of my army ponies. Twi has a lot of my favourite elements about MLP. She's a unicorn, she's magical, her symbol is celestial-based, and she lives surrounded by books. What's not to like?
That episode was called "Too Many Twilights", right?
Twilight Sparkle (as everyone knows by now) was originally based on G1 Twilight, one of the first magic users in MLP cartoon history. She's the Princess of Friendship and one of the most powerful magic users in the G4 canon.
Interestingly, unlike the rest of the Mane 6, Twilight Sparkle did not have a G3 version. The closest G3 version is Twilight Twinkle, a pony who had no significant story line presence, much like Applejack and Fluttershy.
Has anyone else noticed that Twilight Sparkle and Starlight Glimmer share basically the same name? They are extremely similar characters with extremely similar looks too, two sides of the same coin, you could say. Will this become a plot point later on, or is Hasbro just really bad at coming up with original names? (Don't answer that.)