To be fair, I don't think anyone's arguing against that. It's clarified in the OP the definition for a passive/minor characters for this are ones that don't actively contribute to the fighting, since that's the core of DBZ's storytelling. And with that set-up, Yajirobe wouldn't qualify since he was instrumental in defeating Vegeta, whereas Bulma never engaged in any direct combat throughout Z. Everyone knows Bulma's more important, that's why she's the common answer, it's just that the question posed here is really favourite non-fighter in Z so plot importance doesn't discount her.
And yeah, Bulma's the right answer.
