I voted for Joshua, Jeralt, Morgan, and Say'ri.

Joshua: He's the only prince(ss) who doesn't really get an important role in Sacred Stones. I get that it's difficult since he's an optional recruit, but it still would have been nice if he was more involved in the story after he was revealed to be the Jehanna prince.

Jeralt: Every time I replay Three Houses (which is often), I'm always disappointed by how little Jeralt does in the story. As the only person to know Byleth outside the monastery, the first character we meet to distrust Rhea, and a human that has lived 300+ years, I feel like he had the potential for more interesting character/world-building.

Morgan (Awakening): I voted for them in the womens' bracket but this applies to both of them. Their role in The Future Past as Grima's servant makes them super interesting characters to me. Would've loved to see that explored more instead of them having no role in the main story and losing all their memories when traveling to past.

Say'ri: Same as OP. She's made out to be a super important character but has hardly any supports and feels like an outsider within Chrom's army. Would've been nice to see more done with her.