He's not a character so much as a placemarker in his book, is Jeremy, second child of Ross and Demelza Poldark, Regency era Cornwall's sudsiest soap opera couple. The year of Jeremy's birth also sees his daddy on trial for his life, accused of attacking revenuers and inciting a riot during the wreck of a pair of ships near his property one tempestuous Cornwall night, his parents somewhat estranged over some behind-the-scenes meddling Demelza seems to have done to ensure a favorable verdict, his family's fortunes threatened by dastardly deeds of business (by which I mean, mostly, the upstart Warleggan clan), and, as always, his daddy making eyes at Cousin Elizabeth, who was originally set to marry Ross but who married Ross's cousin Francis instead back when Ross was given up for dead in the American Revolution (see the first Poldark novel, Ross Poldark).
For added historical flavor, the assizes at which Ross is tried coincide with a local election, and of course the Poldarks live in a rotten borough, so lots of tasty machinations ensue -- by the way, if you don't want to click on that boring old Wikipedia explanation of a rotten borough, you might enjoy one E. Blackadder explaining it to the Prince Regent:
And yes that's House doing the chicken impression
The machinations in Jeremy Poldark aren't quite as hilarious as those in Blackadder the Third, but they're still a pretty entertaining backdrop to the novel's action.
Other subplots include Jeremy's Aunt Verity finally getting to meet the stepchildren that came along with the marriage Jeremy's mother helped make happen in Demelza, and the somewhat hapless Dr. Dwight Enys, perhaps Ross' best friend, meeting yet another woman who flummoxes him, though admittedly with less disastrous results than the last go-around.
And as I said, Jeremy eventually gets around to getting born, but even his birth is something of a non-event. One hopes this new hero gets more to do in later books...