One of the lesser-known real people who shows up in my alternative history is Richard Wellesley, the Duke of Wellington's oldest brother. In the bibliography of one of my Wellington sources, I found The Eldest Brother: The Marquess Wellesley, 1760-1842 (Iris Butler, 1973). I asked for it from ILL, because I decided I'd be more confident about using him as a character if I read a book or two focused on him rather than just seeing him as a bit player in his famous brother's story.
Reading it forced me to alter my mental image of Richard and to change my fictional plans for him. I'd been predisposed to dislike him, because he tends to come across in Wellington biographies as a bit of a soulless autocrat. I suppose I shouldn't be surprised to discover the truth is a bit more complicated than that, because Wellington himself comes across the exact same way unless you're willing to look beneath the surface.
I ended up feeling sorry for Richard. He was forced to become paterfamilias at the age of 21, when his father died and left him with a mountain of debts and five younger siblings. He worked hard for decades at his political and diplomatic career, in particular during a long term as governor-general of India, only to never quite get the fortune and acclaim he longed for. And then he had to watch one of his own younger brothers, the one who was arguably the most trouble for him to get established in life, get all the fame, wealth, and honor anyone could dream of. That had to rankle, even though AFAICT Wellington was reasonably gracious about the situation. (Reasonable graciousness being the most one could expect from a Wellesley--they were a proud, prickly family with hot tempers they controlled with a varying degree of success, and the brothers had their fair share of quarrels.) Richard and Arthur (i.e. Wellington) were the stars of a gifted family, and were close in terms of ability. I think Arthur was so much more successful because he was far more practical and emotionally stable--though I can't help wondering how much of that was because he had time to grow up and assume responsibility on his own terms, while Richard had it thrust upon him before he was old enough to be ready for it.
Anyway, I had trouble wading through all the political alliances and quarrels in the book, though I'm glad I did, because now I can make my own version of Richard Wellesley richer and more respectful of the real thing.