I've been wondering if you based Ender's personality, leadership style and/or character on any historical persons.
I noticed a parallel between Ender's relentless final battle with the Buggers in Ender's Game and U.S. Grant's battles against Robert E. Lee. Grant was cool under pressure and was willing to fight battles even as his losses were staggering. During breaks in battle, he was overcome with emotion and wept, but then was clear headed and emotionless when it came time to fight again.
Was Ender based on any historical character(s)? Is there one standout character that he is modeled after, or is he a synthesis of characters?
Ender is based on many historical characters. I have read history all my life, with a special eye for military history and great leaders, and I am not surprised that you have found real-world analogues for attributes of Ender Wiggin, as indeed you no doubt could for the patterns shown by other commanders in Battle School and among the adults.
However, no particular historical leaders were used as models, either for characters or for character traits. I relied on my general knowledge to come up with "types" for the commanders Ender worked under and against in Battle School, and while I'm sure that traits of various generals have shown up in the book, it would be far easier to find the commanders who had Ambrose Burnside or Joe Hooker in them than any commanders who represented traits of Grant or Lee. That's because successful commanders make the best of the combination of circumstances, while unsuccessful ones are trapped in their own character so that regardless of the situation they make character-centered blunders. In other words, successful commanders are successful in new ways that adapt to changing circumstances, while unsuccessful commanders fail in all the same old ways.