Being introduced at 30 years of age, Kaji (as he is always referred by) is a handsome man, sporting a ponytail and perpetual stubble clearly visible on his face. During his college years, he and Misato Katsuragi were lovers. That relationship ended before the start of the series though, and they don't reunite until Misato goes to meet Asuka Langley Soryu and they spot each other. When he is brought aboard at NERV, he constantly fights and flirts with her, with her response typically being rejection.
Never losing his cool, even when trouble is apparent, Kaji brings a jolly attitude wherever he goes. His humor is expressed at almost any chance he gets. His calmness is only rivaled by his flirtatious affairs. He is often found flirting with any female of age within earshot, and even though Misato and him are no longer together, she still gets jealous of this.
He plays the role of the father figure in the lives of both Asuka and Shinji Ikari, and both accept him as so, even though it's no secret that Asuka wants him in a different way. Being the calm and collected guy he is though, it's no trouble to play it off without hurting her too much.
Despite appearing to be a happy, care-free person, he is serious deep down. Kaji frequently ponders philosophical ideas and thoughts, and often doubts himself. He believes himself to be a truly lonely individual, and isn't even sure if he can really love another person. This side of him is only brought out when discussing thoughts with Shinji, and briefly in conversations with Minato.
Ryoji Kaji is a triple agent, belonging to more than one group and at different times holding the best interests of each in mind while carrying out orders. He is the one who kidnaps Kozo Fuyutsuki for Seele, but not too long after rescues him for NERV. He really seems to just be working for himself though, doing everything in the interest for uncovering the truth, making use of the resources each organization possesses.
During Kaji and Misato's last meeting, he passed on a small container whose contents consist of data on the Human Instrumentality Project, remarking that it would probably be the last gift he would give her. Afterward, he calls her, getting her answering machine, and leaves a message of farewell, knowing his fate. An unknown character that is never seen onscreen then finds and kills Kaji, with him leaving the assailant with his last words: "You're a little late, aren't you?"