Katya Udinov (affair)
Sergei Semak was the head of Zetrov. Technically, the rightful head of Zetrov should be Alex and upon finding this out, Sergei is doing everything in his power to prevent her from obtaining her rightful place at the head of Zetrov.
Sergei Semek was Nikolai Udinov's right-hand man, and as evidenced in "Guardians", seemed to have a good relationship with him. However, he collaborated with Division and had Nikolai killed, and took control of Zetrov. However, he reneged over his deal with Percy to use Zetrov resources to advance US interests.
In "Partners", when when he discovered that Alex is the one behind Anton Kochenko's assassination he ordered Ari to shoot on sight and send a team to go after Alex, and kills her. A team of Gogol assassins were dispatched to kill her, under the impression that Alex was captured by Division and indoctrinated.
By "Guardians", Ari expresses his confidence that Alex would no trouble as the manhunt for her may have forced her to hide in a Division bunker. He then made suggestions to Semak to keep the black box they're after instead of destroying it, figuring that the information inside it could be used against The US. Semak was hesitant to accept the suggestion, knowing that the black box may contain evidence of his dealings with Division in assassinating the Udinov family. Unbeknownst to Semak, Alex had infiltrated Russia and made her way into her childhood home, as the box was being deliver to him, where she finds her mother alive.
In "Clean Sweep" he visited Ari in Gogol's HQ. They talked about Alex and the black boxes. They have deducted that both black boxes and Alex have secrets about Semak so that both need to be destroyed.
In "Origins" Alex appeared during one of his speeches.