From an interview on Nintendo Dream:
Older compared to the protagonists in other Pokemon games who are around 10, 11.
Male protagonist was drawn by Sugimori to be cooler/hotter looking than past male leads.
Prof. Araragi and her dad:
In earlier designs, Araragi was a fat middle-aged man and his dad was an old man, an aesthetically bad combination.
Thus after complaints from staff, Sumigori changed Araragi to a woman and her dad to look younger.
Araragi is modeled after women working in NY.
Designed to look like an intelligent guy who's earnest and inflexible.
His jacket is made to look like one worn by class presidents to emphasize this.
A girl that does everything at her own pace. Ditzy and not exactly stylish.
Design to give off a "round feel", as female characters in BW are mostly "sharp."
She's especially designed to be the opposite of the female lead, who has a sharp feel to her.
She's older than she looks but calls herself "the dreaming maiden." Roughly the same age as Araragi.
Mostly there as a guide to Game Sync, and so her overworld pixel is a reused/remodel of a female scientist character.
Dreamy eyed as if she's going to drift away, daydreaming.
Shows the dangerous side of being a genius.
Artistic-looking and almost androgynous, he looks like the male lead in some aspects.
A straightforward "hot male" but deliberately shifted away from looking normal.
Kageyama mentions that N's musical theme is a "fragile music box."
Weird. Weird, eerie atmosphere, weird and eerie clothing. Over all a strange, strange man.
Team Plasma and their banners are designed after knights in the middle ages who served their kings.
Despite being triplets, if they looked the same it'll be boring.
Thus their hairstyles were designed to look like the types of Pokemon they'll use.
But apart from their hairstyle, the clothing isn't anything special as not to deviate from the image of a waiter.
One word: big momma.
An African-American woman who encompasses motherhood, and an overall reliable mom.
Her apron serves as both a symbol of motherhood and her job as an archaeologist.
Sugimori was asked to design a cool young man who's also an artist.
After some thought, only his hairstyle was done so Sugimori asked female employees what they see as good qualities in a man.
Eventually he settled on Artie which we see now.
Has long legs compared to others, since she's a model.
Because a fashion models job is to walk emotionless on a runway, turn, then walk back, she's designed to look android-like.
We imagined that she'll be interested in music so we gave her headphones, in return the headphone cables further emphasize her machine-like appearance.
A mine baron and a CEO. Inspired and modeled after Japanese who traveled to the US to pursue the American dream.
Dressed as a cowboy, since he tamed the frontiers of a country that was new to him.
Designed to be the opposite of Kamitsure, and so is short and tough looking.
He's a busy man who wants to get business done his way, so is drawn holding a phone.
Was originally designed as a "big sis" type of character in charge of an all-male crew of airfield workers.
However the "big sis" character type was deemed too out of fashion, and instead she became a cute perky teenager.
Her sexy clothing is mostly what is left over from Fuuro 1.0.
The ball pouch on her thighs are inspired from military issue magazine pouches.
Extra info regarding the leads:
N was designed before the protagonists, so the leads were given caps to match N, not the other way around.
As for the female lead, her short pants came automatically to Sugimori after the cap idea.
After this stage he studied girls on the streets to see what could work well with her cap and short pants.
From here the male lead was designed, to be similar looking to the female lead.
The DP/RS male lead had short hair to match their tame/energetic fashion. However this time around the male lead has rather long hair to reflect his choice of clothes.