This one was a bit harder as I find most of my favorite side characters are male, but after looking through some of the anime I’ve watched, I decided on Ed from Cowboy Bebop.
Ed is a crazy and random little kid, not to mention she looks like a boy with a red version of Tai’s haircut from Digimon, which I’m counting as a benefit for some reason, but she’s also a great technological genius and was even able to easily turn the Bebop into her own personal remote controlled space ship. Pfft. All I got as a kid was a remote controlled truck. 😦
In a show that can sometimes get really heavy in tone, Ed constantly brings a splash of humor and craziness to the Bebop crew, and she also helps them with their bounty hunting through hacking and research.
Her antics are some of my favorite moments in Cowboy Bebop and man I’d love to get ahold of those goggles. Wait….Tai haircut….goggles…..absolutely no other connection besides those two things! *gasp* :0
If she’s not the best supporting female character, she’s definitely the most radical. 😛
Honorable mentions: Agiri from Kill Me, Baby!, Hanji from Attack on Titan, Nice from Baccano!
The Anime Madhouse 