Rabbi Koslov was a Russian Orthodox Jew who lived in the Russian Consortium. A close friend of the Ivanov family, he watched as his dear friend Andrei and his daughter Susan grow apart over the years, especially after the suicide of Andrei's wife, Sofie. As a child, Susan referred to him as "Uncle Yossel." In later years, Susan would seem equally distant from him; referring to him as strictly Rabbi Koslov.
After Andrei's death, Koslov presided over the funeral. A few months later, he traveled to Babylon 5 to assist Susan Ivanova in observation of prayer and forgiveness for the dead. At first Susan (while happy to see him) was cold to his offer, having not yet resolved her feelings for her late father. She was very upset with him when he went to see Commander Sinclair without her knowledge and asked him if Susan could have leave to sit shiva. Before he left, Rabbi Koslov went to see Susan and give what had been left to her by her father. They talked briefly and while he did not directly apologize for interferring in her life, he did get her to open up about how she felt. As he was boarding his ship home, Susan broke down; calling him Uncle Yossel again and asking him to sit shiva.
Rabbi Koslov's sole appearance was in "TKO."