Female lead characters are often used as love interests and miss fanservice. Yuzu is the only one who got fanservice only once. And I'm don't fully agree with your current view on Yuzu. Even though she lost to Sergey to advance the plot, Yuzu plays a major role in getting Sora to the good side. She constantly reminds him that he's now at her and the Lancers' side and that he doesn't belong to Academia anymore. She's convincing enough that even Tsukikage is trusting them. Yeah, she needed help from Sora because she crashed into a building and didn't have a Duel Disk, but once she got a Duel Disk, she and Sora kicked asses. And even without a Duel Disk, she still tries to protect children despite being helpless, which is also a strong point of her character. Yuzu has proven several times that she doesn't need to be a good duelist to be a good character, but the fact that she's a decent duelist and her desire to improve herself makes her even better. I don't remember Asuka and Aki desiring to improve their skills at any point (we got Aki's Riding Duel sub-plot, but we know what happened after).

Speaking of fanservice, here are some memorable fanservice moments:
Anzu: being grabbed my mechanical hands (manga only), bikini moments (both, manga and anime; for those who remember it in the anime, remember Mai being trapped inside a sand/centipede hourglass)
Asuka: hot bath, dress costumes, Harpie cosplay
Aki: Riding Duel outfit, flying in her bathing suit
Kotori: tomato commercial, mini-skirt (so, a walking miss fanservice)
Yuzu: Rin's sleeping clothes