J was a systems administrator in the Structure on a planet in E-Space. He was agorophobic.

J wore a jet-black visor at all times. When the Fourth Doctor, Romana and Adric visited the Structure, he warned them of an encroachment of aliens in the gardens outside.

After the Doctor and Romana explored the Structure, tearing down many of its interior walls, and the gardens, discovering that the aliens were merely insects, they tried to encourage J to venture outside, something he had not done for a long time. He resisted at first, thinking the light from the two suns would be too much for him. He eventually worked his way up to going outside. J tried to help the Doctor figure his way out of E-Space by providing information about nearby stars and planets. The Doctor, Romana and Adric would have stayed longer, but he encouraged them to leave, hoping to reduce his dependency on them. (PROSE: O, Darkness)