“My brother would be capable of standing up to the Russians like that,” his sister tells BBC.
Ukraine has vowed to find the Russian troops who apparently killed an unarmed Ukrainian prisoner of war, after footage appeared online.
“We will find the murderers,” President Volodymyr Zelensky said late on Monday.
Foreign Minister Dmytro Kuleba called for an “immediate investigation” by the International Criminal Court.
In the video, the Ukrainian soldier is seen smoking a cigarette in a trench. He says “glory to Ukraine” before being shot with automatic weapons.
One of the Russian soldiers – not seen in the clip – is then heard saying “die” and using an expletive.
The BBC has not verified where and when the footage was made. It first emerged on social media on Monday.
Neither the Ukrainian prisoner of war (POW) nor the killers have been identified, but the BBC has learned he was in the 30 Brigade.
Russia has not commented.
Kyiv and its Western allies have accused Russian troops of committing mass war crimes since President Vladimir Putin launched a full scale-invasion of Ukraine on 24 February 2022. Russia denies the allegations.
In his video address, President Zelensky said “the occupiers” killed “a warrior who bravely said to their faces: ‘Glory to Ukraine!'”
“I want us all to respond to his words together, in unity: ‘Glory to the Hero! Glory to the Heroes! Glory to Ukraine!'”
Mr Zelensky was referring to a battle cry in Ukraine’s military that has become popular among millions of Ukrainians.
Meanwhile, Mr Kuleba tweeted that the footage was “another proof this was is genocidal”.
Ukraine’s human rights ombudsman Dmytro Lubinets accused Russian troops of committing war crimes by violating the Geneva Conventions regarding the treatment of POWs.
And Ukraine’s Prosecutor General Andriy Kostin said a criminal investigation had already been opened.
Ukraine has previously accused Russian troops of torturing, raping and killing Ukrainian prisoners of war.
Last July, a video emerged that showed a captured Ukrainian soldier being castrated in the Russian-occupied Donbas region in eastern Ukraine.
The Russian soldier filmed carrying out the attack in the town of Severodonetsk was identified as a member of a unit belonging to Chechen leader Ramzan Kadyrov.
In November, Moscow accused Ukrainian forces of executing a group of Russian prisoners.
It followed a video from the front line in eastern Ukraine showing the apparent surrender of a number of soldiers, in an incident that ended in their deaths.
A Ukrainian official said the surrender was “staged” by Russian troops in an attempt to attack their captors.