Isis was involved in the 1968 sabotage and detonation of a nuclear missile in the atmosphere just 104 miles above the surface of central Eurasia. This near-calamity made the rival governments cease, for a time at least, the deployment of nuclear weapons in orbit, and rethink their old strategy of "balance of power".
Though apparently Seven's pet cat, Isis was more: she communicated with him through telepathy, and understood his spoken words. She accompanied him wherever he went and watched out for him. Isis was also able to take Human form. In both forms, she wore the same collar around her neck. (TOS: "Assignment: Earth")
Isis was played by three cats, one of them named Sambo.  Her purrs were provided by Barbara Babcock, who received no credit for the voice performance.  Isis in her Human form was played by April Tatro. 
According to the Star Trek Encyclopedia (3rd ed., p. 215), "Isis was apparently intended as a continuing character in the proposed series Assignment: Earth."
In the Eugenics Wars novel The Rise and Fall of Khan Noonien Singh, Volume Two, Isis was sent by Gary Seven to spy on Khan Noonien Singh's "Great Khanate", based out of the city of Chandigarh in northern India. She assumed the name "Ament" (the name means "one who is hidden", or "hidden goddess" as Seven put it) and became one of Khan's most trusted advisors, while at the same time providing intelligence of Khan's operations to Seven and Roberta Lincoln. After Khan was defeated and brought to the Botany Bay, Isis was killed when she stepped into the path of a knife (thrown by Khan's bodyguard, Joaquin) intended for Lincoln.