Cassandra Ovechkin is an old flame of Michael's.
Six years ago in Belarus, Michael was on an undercover mission, romancing with Cassandra. He later reveals his true identity to her and ask for her help to take down the Belarusian president Valeri Ovechkin, her husband. Cassandra helps Michael and Division and a replacement look-a-like takes over the spot as president.
Even though Michael does not appear to hold any romantic feelings for her, he does care for her deeply. She apparently reminds him of his previous wife and child. Cassandra's son, Max, is actually Michael's son even though she denies it. Nikita is aware of this fact and told Michael about it at the end of the "343 Walnut Lane" episode.
In "London Calling" Michael visited her and Max in London. General Tupelov kidnapped her and his men pushed her car, with her in it, off the ledge. She survived the fall and killed those men. She revealed to Michael and Nikita that she works for the MI6 since the first operation started in Belarus.
In "Arising", Ari Tasarov assigned Cassandra to trick MI6 into thinking that Gogol had acquired aerial drone designs, promising that it'd be her last mission. What she didn't know was that Ari was planning on framing her team for the death of Sergei Semak. While one of her team began tailing the Gogol courier, Cassandra spotted Nikita on the street cameras and left to investigate. However, she was ambushed by Michael, who had just discovered her true allegiance. While Cassandra insisted that this would be her last mission, Michael deduced Ari's true plan and stopped it. But Ari in turn deduced that Cassandra betrayed him and leaked her identity to MI6, causing her to team to turn on her. After Nikita extracts Max before MI6 could use him as leverage, she then faked Cassandra's death. Micheal instructs Cassandra to "disappear" with Max and decides to stay out of contact with them for their sake.