Patrick

From BioShock Wiki

Patrick is Atlas' non-existent infant son. Atlas frequently mentions Patrick and his also-fictional wife Moira to Jack in order to gain his trust and sympathy. To further expand his trust with Jack, he pretends that Andrew Ryan killed them in a submarine and laments heavily. His goal from that point of the game until his identity is revealed seems to be revenge, with the help of Jack.

There are also posters scattered about Rapture for a play called "Patrick and Moira", from which Atlas/Frank Fontaine likely got his inspiration for the fictional wife and child.