Joseph Sullivan is Samuel Sullivan's deceased brother. His funeral was held in the first episode of the fourth season, with Samuel conducting a eulogy and then a burial.
Joesph has the power of Pathokinesis, meaning he can change the mood/emotions of someone and also feel them. (SPOILER ALERT ) In the middle of the eleventh episode of season four titled "Thanksgiving," Samuel confronts Joseph about the words of Dr. Suresh. Joseph tells him, "You need to calm down, brother." After that, he said, "You stop controlling me. Stop manipulating my emotions." These words imply far more control than normal human interaction. Joseph replies, "Fine, you want me to let go? I will." This last piece of evidence tells us that it is not something as general as background manipulation through actions and words but rather something that can be started and stopped at any moment.
He was killed by Samuel by having a rock impaled into his throat.