Alina Rostoff was G. Callen's foster sister when he was staying with her family. She was four and he was fourteen. Despite this, they had a very close relationship and Callen's stay with the Rostoff's was an important part of his mysterious past.
During his time in the DEA, Callen was involved in an operation codenamed Cossack which involved him and a team of agents to stop an oil deal which would place control of Russian oil shipments towards Europe into the hands of the Russian Mafia. The op was betrayed and, ten years later, Callen, Ethan Stanhope from the CIA and Ricky Taylor from the DEA were shot. Stanhope and Taylor died but Callen survived.
Arkady Kolcheck, the Russian control officer for Operation Cossack, was also targeted but they failed to kill him. Before the attack, Arkady discovered that someone was trying to kill them and tried to warn the others. He asked Alina, whom he knew about from Callen, to meet with her foster brother in LA and tell him. She tried twice, once when he and Leroy Jethro Gibbs were catching up and the second time just before the assassination.
She went to Arkady and told him that she wanted to avenge Callen's death (not knowing that he was alive) and he told her that he suspected a business man named John Cole of being behind the attack. Alina was trained in surveillance by Arkady but was killed the day after she started her surveillance.
Callen and the OSP started to investigate her death and they discovered everything. Soon, it was revealed that the traitor was Ethan Stanhope and that he faked his death so that the CIA wouldn't find him or draw a connection to him. He was also planning to kill Arkady but failed twice.
His henchman, Max Tyrus, killed Alina when they discovered she was watching Stanhope. Stanhope was arrested and Tyrus was killed.
After all this, Callen went to Alina's house where he had spent his three months with them and relived good memories.