Pte Matthew Boyd, from the Royal Gibraltar Regiment, was found injured and unconscious in Brecon early on Sunday morning and later died.
Two men have been arrested in connection with his death.
Elaine Mawhirk said Matthew had always wanted to be a solider and had joined up when he was 16.
"Because he was so young his mum and dad had to sign a form to give permission for him to sign up," she said.
"He was just about to renew his four-year contract with the Army.
"He was doing very well, he was progressing so well within his career with the Army."
Pte Boyd and his family lived in Gibraltar but he was originally from Northern Ireland, where some of his relatives still live in Carrickfergus, County Antrim.
Mrs Mawhirk said her nephew had been on a training exercise in Brecon, before the incident.
"He volunteered because they were short of men for the training exercise and Matthew put himself forward to go and assist in the training of new recruits in Brecon," she said.
"That was just his love and his passion. He had so many friends and support within his regiment.
"The messages we're receiving and reading the testimonials from his regiment in Gibraltar is just heartbreaking.
Matthew's aunt said his death had left the family "devastated".
"Matthew was fun-loving, lived life to the full, he had an amazing wee life and packed so much in at the age of 21," she said.
"I'll remember his cheeky grin.
"As a family growing up we spent all our holidays together in Spain and no holiday is ever going to be the same because we're just going to be missing Matthew.
Welsh police have arrested a 23-year-old man on suspicion of murder while a 22-year-old has also been detained in connection with his death.
Both men remain in custody.
Dyfed-Powys Police said Pte Boyd was wearing civilian clothing when he was found and the 23-year-old who was arrested was not a serving soldier.
The aunt of a soldier born in Northern Ireland who was killed in Wales has said he was "fun-loving" and "lived life to the full".