A charming sociopath, Keller is extremely cunning and adept at manipulation. Throughout the series, he has a complicated, intimate relationship with fellow inmate Tobias Beecher. Keller has been married and divorced four times. As a young man he spent time in prison where he met, and had a sexual relationship with, Vern Schillinger.
Keller is frequently presented as immoral and untrustworthy, manipulating and destroying anyone who tries to get close to him. He has a strong desire for dominance in any relationship and often chooses partners based on how he can treat them and still have them love him. Despite his sociopathic tendencies, Keller is a more complex individual. He appears to feel genuine love for Tobias Beecher but cannot express it and is very well aware of how despicable his actions can be. He has deep self-loathing, once telling Sister Pete that he is "worthless, as bad as they come."