Kaj, of the House of Tus'tai, was a member of Klingon Imperial Intelligence. A member of the service's active operations division, by 2300 she held the rank of major.

Kal had one sister, Kol, who died in 2300 when Kriosian terrorists detonated an isolytic subspace weapon near Da'Kel. Because Kal had been rendered infertile by an incident earlier in her career, the death of Kol meant that the House of Tus'tai would die with her.

When General Igdar refused to listen to the Federation's findings, Kaj allowed Starfleets' Lieutenant Elias Vaughn, Commander Darius Miller, and former Lieutenant Valeris to accompany her to help stop the Kriosian terrorists from continuing their campaign of terror.

Prior to taking them aboard, Kaj learned all she could about Valeris, including the trauma the Vulcan had suffered as a child at the hands of corrupt Klingon officials. Kaj was able to accomplish what Federation medical personnel could not in helping Valeris to finally put her hatred of the Klingons aside, telling Valeris that she was judging all Klingons based on the actions of those who had terrified her as a child, whereas Vaj judged humans based on men like Darius Miller who had proven their strength and honour.

After preventing the detonation of another isolytic weapon over Qo'noS, Kaj placed General Igdar under arrest and dragged him in front of the High Council to answer for his many crimes and attempted overthrow of Chancellor Azetbur.

She then went on a mission to assassinate the members of the Tal Shiar who had supported the Kriosian terrorists, including the Tal Shiar agent once known as Nanclus. (TOS novel: Cast No Shadow)

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