If I had to choose then it'd probably be J. Rock Champion.
Reading about his exploits in
The Siege of Lexington, Missouri: The Battle of Hemp Bales by Larry Wood was one of the most enjoyable parts of the book, and he seemed like quite the character.
I have no familial relation to him, however.
For a good coverage of his actions, there's this link,
The story of Rock Champion, Missouri Minute Man and Confederate soldier.
www.civilwarstlouis.com
and I strongly recommend reading the Battle of Hemp Bales book overall.
This may have been more interesting if it was set for your
favorite soldier on both sides!
Excellent idea, however my favorite Union
soldier was a
sailor, Douglas R. Cassel, and if we're including Navymen then it'd be a very touch choice for me between Isaac Brown and Rock Champion.
In regards to Cassel, I visited his grave in Zanesville, Ohio on the same roadtrip I visited the Wilson's Creek National Battlefield on. Photo bellow,
Cassel fought at Mobile Bay and the Second Fort Fisher, however what I find most interesting about him is his post-war service.
He participated in the 1871 Korean Expedition, commanding artillery, and was hired by the Japanese government as an advisor for their 1874 Japanese Expedition to Formosa. He essentially commanded the Expedition in it's first few days and negotiated with the Natives. He (or James Wasson) also brought a Winchester along the Expedition.
He sadly died of malaria he caught from the Expedition, as far as I know the only non-Nisei American to die in Imperial Japanese military service.
For reading I recommend
A Conspicuous Ornament: The Short, Eventful Life of Lt. Cdr. Douglas R. Cassel, U.S.N. (1845-1875)
By Roger D. Cunningham.
As with Champion, Cassel and Brown are not in my family.