Craig Mackintosh, a British national, was killed in Ukraine where he was volunteering as a medic, his sister announced on Thursday.
Lorna said her brother was killed on Aug. 24.
She set up a crowdfunding campaign to raise £4,000 to bring his body back home. Local media reported that the crowdfunding campaign has already exceeded its target.
"Please help us bring this war hero home," she wrote in her appeal.
"Our brother bravely volunteered to go to (Ukraine) as a medic to help save lives in this war-torn country," she said. "In the line of duty, helping others he lost his life."
Lorna asked for help to bring her brother, who she said was stranded in a morgue in Ukraine, back to the UK.
"He gave his life to save others and he needs to come back home to have the service he deserves," she added.
A British Foreign Office spokesperson said: "We are supporting the family of a British man who has died in Ukraine and are in contact with the local authorities."
In June, Mamuka Mamulashvili, the commander of the Georgian Legion, a military unit under Ukrainian command, told Sky News that there were around 20,000 foreign fighters fighting alongside Ukrainian forces, with around 3,000 being British.
Also in June, Aiden Aslin and Shaun Pinner, two British citizens captured fighting alongside Ukrainian forces, were sentenced to death by a court in the self-proclaimed Donetsk People’s Republic in eastern Ukraine.