Gideon Cahill was the founder of the Cahill Family.
Gideon Cahill was the original Cahill. He is the father of Luke, Katherine, Thomas, Jane, and Madeleine Cahill. Damien Vesper was Gideon's friend before 1507. In 1497, they were celebrating a feast when Gideon dared to correct him on a point of astronomy. After the feast, the two of them talked deeply about the point.
In 1507, Gideon drank the master serum, which was not perfected by the Madrigal Clues. He gave Luke, Katherine, Thomas, and Jane a part of that unperfected serum, each, after a fallout with Damien Vesper. During the fallout Gideon used the powers granted by the serum, and realized the serum would kill him, unless he perfected it. Gideon was killed in a fire soon after, which everyone else thought was set on purpose, by their brother Luke, which lead his children scattering around the world, and founding the branches of the Cahill family. The actual reason for the fire was Damien Vesper, although Gideon technically started it (to save his family/stop Vesper). Nobody knew his wife Olivia was pregnant with Madeleine Cahill at the time.
Gideon Cahill's hair is described in Vespers Rising as a swirl of wild gray tufts like a bank of storm clouds. His rugged face was darkened from years of working in a laboratory. Chemicals had turned his frock into a palette of stains, and his forearms were covered with notes in Latin - notes he wrote when he couldn't find pieces of parchment. Gideon's gold ring, according to Damien Vesper marked him as a man of worth. His eyes were fierce and bright under those bushy gray brows, too.
Olivia Cahill- wife
Luke Cahill- Son
Katherine Cahill- Daughter
Thomas Cahill- Son
Jane Cahill- Daughter
Madeleine Cahill- Daughter (Born after he died)
Winthrop Cahill- Grandson
Toyotomi Hideyoshi- Grandson
William Shakespeare- Great Grandson
??? - Mother (Lived in Milan)
??? - Brother (Lived in Dublin)
Madeleine the Matriarch - Great Grandmother
Mary Arden - Granddaughter
Keiko - Daughter-in-law