Gregory Illéa is a historical character often mentioned in The Selection Series.
Gregory was the founder and first king of Illéa. At the beginning of the series he was already long dead and his country was ruled by his great-great-nephew (by his wife's family, but he is also technically his great-grandson by his ex-daughter-in-law), Clarkson Schreave.
Nothing was mentioned about Illéa's childhood and time growing up, as America Singer only read his diary entries during the onset of World War IV. At that point, Illéa mentioned that he was an ideal person people looked up to, because he was rich and had a beautiful family, both of which he worked very hard for. He was shown to be ambitious and decided to use his money to gain influence in the military and politics.
Another entry explained his disdain for Halloween, and he wrote up a draft of the caste system in one of his entries. Eventually, the entries showed his ambitious side as he plotted to overthrow the president without looking like a usurper. He also showed how he did not really love his family and only used them for his image. Gregory eventually gained control of the country and he implemented the caste system, assigning his friends and contributors in high castes while placing his enemies in the lower castes.
The last entry mentioned in the novel explained how his daughter Katherine's wedding day was a political move for him, as he forced her to marry Emil de Monpezat, the Crown Prince of Swendway in order for him to become the King of Illéa. It was implied that Katherine was shipped to Swendway with her husband, eventually crowned queen, and had a daughter named Antonella.
Gregory's eldest son and heir, Spencer, died after a few years following his sister's marriage. Damon, Gregory's youngest child, then became the next king. After the death of Spencer, and Illéa's wife, Bethany, the country had low morale, and thus the Selection was created to appease the public. The first Selection was a setup and Gregory made Damon choose Grace Lowell, a Two, after Damon slept with half the girls.
Damon eventually had a son named Justin, who held his own Selection and chose Abby Tamblin, a Three. Despite being fond of Justin, Abby was in love with Justin's cousin, Porter Schreave, and the two conspired to kill Justin. Illéa was aware of this, but knowing the Illéan royal line would end, he convinced Abby to marry Porter. Illéa died before Abby and Porter's son, Clarkson, was born.
America learned from these diaries who Illéa really was and how the nation was founded. During a speech, America revealed her knowledge of the diaries on live television, resulting in Prince Maxon getting caned by his father, Clarkson.
While Gregory was never actually seen in the books, his diaries show that he was ambitious since he went through a lot of trouble to get to his spot of power. Gregory also seemed to have been uncaring about love and family. He married off his daughter to someone she didn't like, his wife might have purposefully killed herself after marrying him. After his grandson died, he made his granddaughter-in-law marry Porter Schreave to continue the royal line.