Despite a turbulent career that constantly calls for him to be away, Castle considers himself a devoted husband and father. He often uses thoughts of his wife, Maria, and their two children, Frank Jr. and Lisa, to carry him through tough times during combat missions. In fact, it is the thought of getting home to them that motivates him to become a one-man killing machine during an ambush in Kandahar, during which he singlehandedly rescued the remnants of his unit.
Castle’s love for his family is so strong that their abrupt and violent deaths drive him into a fugue state where he can’t see any alternative but to kill every last person who had a hand in their murder.
During this time, Frank essentially cuts himself off from any human emotion that isn’t rage or despair, with only legal aid Karen Page, who works for the defense firm Nelson & Murdock, getting through to him and seeing his inner humanity. She is one of the very few people he makes an exception for, and he will even drastically deviate from his plans to make sure she is safe and free from harm. He even remarks that he sees her as family, exemplifying how deeply he feels for her.
After Castle kills off everyone he thinks is involved with his family’s death, he’s contacted by an off-the-grid former NSA analyst named David Lieberman, AKA Micro, about the true nature of his actions as part of Operation Cerberus.
Though their partnership is relentlessly rocky at first, Castle eventually comes to think of Micro as a trusted ally and friend. He even befriends Lieberman’s family — wife Sarah and children Leo and Zach — and, for a short time, considers them his own surrogate family. With Sarah and the kids under the belief Lieberman is dead, Castle supports them, albeit briefly.
Castle also remains friends with Curtis Hoyle, who he still feels guilt over due to an incident where his hesitation on the battlefield caused Curtis to lose his leg.