Ee'char, an Argrathi prisoner and Miles O'Brien's cellmate during his artificial memory incarceration on Argratha, was a humanoid that only existed in O'Brien's implanted memory.
Ee'char, who had been imprisoned for six cycles (since 2366), helped O'Brien adjust to prison life during his 20 cycle term. During his previous six cycles of solitary confinement, Ee'char exercised, told himself stories, and drew Eseekas to keep his mind sharp. He later taught the art of Eseekas to O'Brien. He also taught O'Brien how to save food, because he said that sometimes the guards would forget to feed the prisoners.
Twenty cycles into O'Brien's artificial memory, and only one or two weeks before he was released, O'Brien killed Ee'char, thinking that he was hoarding food from him. In actuality, Ee'char was retrieving a second stash of food that he was going to share with O'Brien – a fact O'Brien realized only after Ee'char was killed.
After his release, O'Brien had a difficult time readjusting to real life. He was haunted by Ee'char and what he had done to him. He imagined seeing Ee'char around the station, although he knew the alien wasn't real, and even had a conversation with him while sitting in Quark's. Ee'char encouraged Miles to open up to his friends, and to accept their help in recovering from the emotional trauma his incarceration caused. In the end, Ee'char seemed to forgive Miles for his murder, smiling warmly and wishing for Miles to "be well" before disappearing forever. (DS9: "Hard Time")