General Tupelov was the man Cassandra owed money to.
In "London Calling" Michael and Nikita payed him a visit and asked him to leave Cassandra alone. They've beaten up all of his men and Michael started punching him. Nikita stopped him. Later he kidnapped Cassandra. He told him men to push her in a car of the cliff. She survived the fall. Later Nigel came and shot him.