The Sole Witness of The Ultimate Showdown of Ultimate Destiny (we're going to call him Dude just because) was the sole witness of The Ultimate Showdown of Ultimate Destiny. There is nothing else notable about this man in any way, except he won The 2nd Showdown of Ultimate Destiny. He doesn't have a job, family, or life. The only thing he ever did was watch The Ultimate Showdown of Ultimate Destiny, and won the Second One.
Dude was born somewhere in the UnWorld at some time before The Ultimate Showdown of Ultimate Destiny. He grew up as a total Nobody that Nobody knew about. A bunch of stuff happened, and then The Ultimate Showdown of Ultimate Destiny happened. He just happened to be hanging out in Tokyo city at the time, so he saw the whole thing.
He watched for a hundred years as the showdown raged on. He just stood there and watched. As the battle got more intense, Dude's sanity began to slip. By the end of the battle, he had totally lost it. He is now locked in some madhouse, until Thanos snapped it away and he somehow survived it. After that Universe was destroyed, he rebuilt it out of Ice Cream.