Was wondering the same thing when I finally got around to 'Heart's Missing Piece'. It's that manga by Seo Kouji, right? The main pair are Saki and Haruto. Honestly, Saki kinda grated on me for the first half—she’s the rich, sheltered girl who gets into this arranged marriage with Haruto, who’s basically the family butler. Their dynamic is the whole engine of the thing; she’s all impulsive and emotionally messy, he’s trying to be this stoic, dutiful rock while clearly being head over heels. There’s also Saki’s dad, who’s the classic controlling patriarch setting up the whole mess, and Haruto’s younger sister, Miho, who shows up later and adds some much-needed perspective. I kept reading mostly for Haruto’s slow burn. Seeing him carefully dismantle Saki’s defenses, bit by bit, was way more interesting than her initial tantrums.
Don’t expect a huge sprawling cast. It’s really a two-hander for most of it, which works for the claustrophobic, pressure-cooker vibe of their situation. The side characters exist mostly to reflect different facets of their central conflict.