Sigma has been in a coma for 3 months. When he woke up, Nikolai hugged him tightly.

-"God, I thought I lost you forever..."

He started crying, since Sigma had to remember things since he freshly woke up, when he did, he was surprised that Nikolai was crying.

-"I only could save one, not two."

Turns out, he saved Sigma over Fyodor because he just loved Sigma more.