Serious characters often get a bad rep for being boring but what sets them apart from their counterparts is the fact that they're usually the smart ones, not smart in a geeky/technological way which I differentiate as "intelligent" but smart as in a down to earth, street level smart. the serious archetype tend to be reserved for leader status for a reason, these are some of my favorites.

Leonardo from the 2003 TMNT: I feel Leo in general has always had a bad reputation, it seems where ever I hear someone's favorite turtle it's never Leo, for me I always loved Leonardo for the same reason I like these other characters, at the end of the day they're the leader because they can beat the rest of the team in a fight, for one Leo has a weapon that can actually kill someone, Raph does too but his weapons being shorter provide a significant disadvantage from the gate, he may be strong and tenacious but Leo is calm and quick witted, prefering to use the environment to get the upper hand, that and swords are way cooler as well as the color blue, name something else that's cool and blue, Sub-Zero motherfucker.

Huey from The Boondocks: I have always loved Huey, his counter culture attitude, his down to earth personality, he has some of the best episodes of the show and despite being a child is always the voice of reason, the funny thing is nobody listens to him and when they do things go bad like in the movie theater episode. I also just love it when he beats the shit out of Riley and literally gets his ass kicked every where else, Bushido Brown, Luna, Stinkmeaner's crew, they all just lay into this kid. And Huey just feels real, goes to show how strong the writing used to be on the show before it got shot to shit, the writing experienced a "Nigga Moment" so to speak which is the best criticism of season 4 honestly.

Frylock from Aqua Teen Hunger Force: Frylock probably has the weirdest design out of the 3, he is a floating McDonalds french fries that shoots lightning from his eyes, has a jewel on his back, has braces and most importantly.... that bitchin' goatee. It's so funny to me that someone as intelligent as him hangs around with people like Shake and Meatwad, it's cruel but he never seems to resent his position as head of the house or resent his roommates.
Also I love it when every thing explodes when they throw stuff and you can thank Frylock for that one.

Blossom from The Powerpuff Girls: Blossom gets the same hate Leo gets as for their crime is being boring, I mean they're all little girls in this case so their character doesn't go passed being one dimensional but Blossom is cool too, for one she has the strongest design, think about the other girls, Buttercup is green and black, Bubbles is blue with blonde hair, Blossom is pink and has orange hair but what sells it the most is her red bow that gives a nice complimentary color and a more unique silhouette, black out the other two and they'll be harder to tell apart. I don't know I just like Blossom a lot.

There are other characters I like but these are the ones I immediately think of.