Now that I'm thinking about it, I'm realizing that Yuugi, Kaiba and Jounouchi feel challenging to write in specifically similar ways because they are tension-based characters. Yuugi's sweet and gentle nature vs. his anxious obsessiveness and buried anger, Jounouchi's 'golden retriever energy' & intense steadfast loyalty vs. his predilection for impulsivity, temper and sometimes violence, Kaiba's...well, innate Kaiba-ness, vs. "my life goal is building theme parks for kids" vs. the spectre of Gozaburo biting at his heels at all times vs. Mokuba vs. whatever remains of an ancient priest in his soul that's also attached to the soul of a huge powerful dragon, etc. etc.
It's not that straightforward characters like Anzu aren't challenging in different ways. Anzu knows roughly who she is, what she wants, and what she believes, with some very normal teenage wibbly-wobbliness. The challenge in writing her comes from sorting through the mire of early-aughts baked-in shounen misogyny and excavating the (very good!!) character buried in there, and figuring out how to extrapolate the kind of development for her that the other characters received.
Yuugi, Jounouchi, and Kaiba did receive a lot of that development and so less extrapolation has to be done, but also none of them ever really "resolve" in the sense that the fundamental tension of them goes away. It persists through character development, because it's a different thing. It's not a lesson to learn, or even necessarily a problem to fix. The tension is the character. Developing any of them doesn't look like unifying them into a perfect, cohesive whole; but instead exploring any one of hundreds of different pathways for what holding and managing that tension looks like across a single story, a long arc, a lifetime, so on and so forth.
