The Duke of Soleanna featured as a character in Sonic the Hedgehog '06. A scientist and statesman, the Duke ruled the nation of Soleanna ten years before the main events of the game's storyline, and was also responsible for the Solaris Project, an errant experiment in temporal manipulation that ultimately led to his death. He is the father of Princess Elise the Third.
What with being dead in the "present" timeline of Sonic '06, the Duke has only a few in-game appearences; once when Shadow and Silver travel back in time to observe the Solaris Project firsthand, once in Elise's flashback in the cutscene following Tropical Jungle, and once at the end of the game, when he and the young Elise are viewing Solaris in its early form - the Flame of Hope - before Sonic and the present-day Elise extinguish him.
After the death his wife (Elise's mother), the Duke resolved to use the power of Solaris to travel through time and see her again - as well as allow humanity to correct its past sins. However, the Project ended in tragedy with a "spatial meltdown", causing Solaris to split into two villainous entities: Iblis, the raw power, and Mephiles, the conscious mind. The Duke was gravely wounded in the explosion while attempting to shield Elise. Upon the arrival of a time-traveling Shadow and Silver, the hedgehogs both split up to pursue and capture Mephiles and Iblis. The Duke, strangely not surprised to see them, gives Shadow the Scepter of Darkness in order to imprison Mephiles, and uses a Chaos Emerald to seal Iblis within Elise's soul just before succumbing to his mortal injuries.
Because Sonic and Elise set up a paradox by extinguishing the nascent Flame of Hope at the end of the game, the Duke cannot have been killed in the Solaris Project, so he may still be alive in the "final" version of the timeline.