Cartier St. Croix is the son of Louis St. Croix. He is a rich and eccentric man, a wealthy former president of several corporations from Monaco, and an ambassador. As his father before him, Cartier joined the Xavier's Mutant Underground.
He married a descendant of Algerian royalty, and she bore them a son, Marius. Their second child was a daughter Monet, who quickly became Cartier's favourite child. A few years later his wife gave birth to twins Claudette and Nicole.
Claudette suffered from autism, and despite Cartier spending large sums of money on her treatment, her condition would remain unchanged.
While Cartier was out with his daughters, Marius' mutant powers manifested for the first time. His wife tried to help their son, but overcome by his newly awakened hunger, Marius killed her. When Cartier found out what his son had done, he angrily kicked his son out.
Sometime later, Marius returned, to offer Monet a place at his side. Monet scornfully rejected him, prompting Marius to turn her into a mute, razor-skinned creature called "Penance". The twins caught Marius in the act, and banished him to a hellish dimension, dragging Penance with him. Afraid of what would happen to Cartier if he found out what had happened to his beloved daughter, the twins began posing as her by using their mutant powers to merge into her.
When Monet was freed from the Penance body, Cartier came to the academy to visit his daughters. When he learned that Marius, who had been captured by Generation X, was there as well, a remorseful Cartier attempted to make peace with his son, believing he had been rash to throw him out. Marius refused to reconcile with his father however, and appears to have escaped the academy before Cartier could make any headway in reconciling them. 
In an effort to bring his family closer together, he decided to take his daughters home. He sent Monet to study in Switzerland, but when she asked to return to the academy, Cartier allowed her to.
When Monet attracted the attention of Baron Mordo, who suffered from cancer and needed to drain Monet's life essence in order to be cured, he kidnapped Cartier in order to force Monet to cooperate with his plans. Monet, with the help of Strong Guy, managed to rescue her father, and used her telepathy to trick Mordo into believing that he had drained her. After this ordeal, Cartier seemingly returned to Monaco.
- 8 appearance(s) of Cartier St. Croix (Earth-616)
- 2 minor appearance(s) of Cartier St. Croix (Earth-616)
- 6 mention(s) of Cartier St. Croix (Earth-616)
- 2 image(s) of Cartier St. Croix (Earth-616)