From "Star Trek: Day of the Dove (#3.7)"
[as Chekov is going after the Klingons armed with a sword, Kirk tries to hold him back
Don't try to stop me, Captain. I saw what they left of Pyotr. And I swore on his grave I would avenge his murder.
[enters the turbolift
What's Chekov's grudge against the Klingons? Who's... Pyotr?
Captain James T. Kirk
His only brother, killed in a Klingon raid.
His brother? He never had a brother. He's an only child.