Ah, yeah, but she doesn't fit into the stereotypical magical girl image! XD
Ok, fine. How about Ami Mizumo. AKA, Sailor Mercury.
It's just a consequence of the "5 man band" structure. Every 5 man band needs a leader, a lancer, a smart "guy," a big "guy" and a heart. And, in that arrangement, either the lancer or the smart "guy" are usually going to be the serious type.
(For clarification, I'm only digging into this because I have a compulsive need to correct inaccurate statements.)
The only thing that holds magical girl arrangements as being any different from the equivalent arrangement in super-hero or adventure arrangements is that in a 5 magical girl band the leader has a tendency to have the personality of the heart, and the heart has the tendency to be replaced with some kind of deva / performer type role.
In the example of Sailor Moon, the poster child for the magical girl genre, it would be Usagi/Sailor Moon as the leader, Ami/Sailor Mercury as the smart "guy," Rei/Sailor Mars as the lancer, Matoko/Sailor Jupiter as the big "guy," and Minako/Sailor Venus as the Diva (AKA what would have been the role of "heart" in anything other than a magical girl series.)
EDIT: I just realized while doing the quick research for the names here, but all of their names reflect their powers. Ami Mizumo has water powers as sailor mercury, Rei Hino has fire powers as sailor mars, and freaking Usagi Tsukinoko has moon powers as sailor moon.
(Mizu is Japanese for water, Hi (pronounced "he") is Japanese for fire, and Tsuki is Japanese for moon.)