Asking this question implies that a character, or a plot, will be one-dimensional. Flat. Maybe boring.
Supporting characters can be naughty or nice. The fumble-fingered side-kick. The dotty neighbor. The all-wise grandfather. The ultra-brave (or super-silly) maidservant. The sell-out brother-in-law. A Dr. Watson. Their role is not to be more interesting than the lead.
A romance is constructed around the main characters, and how they change so they can have a happily-ever-after.
I didn’t discover my first heroine’s depth until I was writing the second to last chapter. She’d just returned to her family’s South Dakota ranch after she’d summoned the inner strength to escape from her kidnappers and collapse in the arms of her lover, who’d turned heaven and earth inside out to be in the exact right place to catch her.
I thought I’d resolved all the subplots, and all that was left was to reunite them for their HEA. And I was unable to script that path. She could plausibly morph into a stoic spinster. She couldn’t say oops and rush back to him without hurling off some pillar principles.
Then a secondary character demanded his due. He found my heroine in the loft of the barn. She was sitting where she and the hero had first made love, where the hero had given her a ring (and meant it), and where he’d had to confess that the ring was bought for another woman.
The secondary character revealed a secret that filled the core void in her identity. She learned her mother had been naughty as well as nice. Her mother had loved him as much as the man the heroine believed was her father. He explained that love has many forms of expression, and although she wasn’t the hero’s first love, she could be his forever love. If she accepted he had a complicated naughty, but nice past.