It would seem that Christ is talking about the Devil here or is He talking about our very selves.
Christ may have been referring to the rabbinic duality of yetzer hara, the so-called "evil inclination," and the yetzer hatov, the "good inclination,". Yetzer hara is not a demonic force that pushes a person to do evil, but rather a drive toward pleasure or property or security, which if left unlimited, can lead to evil (cf. Genesis Rabbah 9:7). When a person’s will is properly controlled by the yetzer hatov, the yetzer hara leads too many socially desirable results, including marriage, business, and community. In Judaism adults are distinguished from children by the yetzer hatov, which controls and channels the drives that exist unchecked in the child. Thus children may seek pleasure and acquisition, but they are not able to create a sanctified relationship or exercise the responsibility to engage in business. The young adult is not described as someone who has developed a sophisticated moral sense; in fact, the early adolescent may base moral decisions entirely on fear of punishment. Yet by age 13, the child’s moral sense has developed sufficiently to hold the child responsible for his or her actions.
Another Jewish source states: