The school friend, who did not want to be named, described Abdul Raqib Amin as a genuine guy who had fallen in with the wrong group of people.
He said: "I think he has been brainwashed. The man in the video is not the Raqib I know."
He said he last spoke with Mr Amin around two years ago, when he was preparing for a pilgrimage to Mecca. The friend said he believes it was during this time that Mr Amin became radicalised.
He said: "He was always really impressionable but just fell in with the wrong crowd. He was normal but easily led. He has met someone who has taken advantage of him."
Mr Amin, 24, was identified after appearing in a 13-minute video alongside two fellow British Muslims, from Cardiff, urging their compatriots to fight with Sunni extremists Isis in Syria.
Isis, an off-shoot of al Qaida, has mushroomed in the Middle East in recent years, finding a foothold in war-torn Syria and more recently seizing control of Iraqi cities.
Mr Amin, 24, was described as "a good kid" and a "great neighbour", as members of the local Muslim community spoke of their devastation that the city was now linked to the Isis terror group.
Aberdeen Mosque spokesman David Harrel said: "Aberdeen Mosque and Islamic Centre asserts unequivocal condemnation of all these activities based on religious values and our identity as Scottish and British Muslims.
"No political or religious cause could ever be assisted by such acts of incited terrorism."
Mr Amin is understood to have moved to Scotland from Bangladesh when he was around 10. He grew up in the Froghall area of Aberdeen and attended Sunnybank Primary and St Machar Academy. It's believed he later moved to Leicester and his parents are thought to have returned to Bangladesh.
Maqbool Choudry, who knew Mr Amin for a few years before he moved to England, said: "The Imam always preaches against this. He always tells - especially the youngsters - to keep away from things like that.
"I've been in Aberdeen for over 20 years and I've never ever come across anything like this before. That's why the whole community is devastated and shocked."
One former classmate said Mr Amin had "suddenly changed" a few years ago. He said: "He seemed like a nice guy. How has somebody from Aberdeen managed to get involved with something like this?"
Mr Amin's former neighbours Michael Hind, 65, and wife Barbara, 61, who live in Froghall Avenue, Aberdeen, confirmed it was him in the picture.
Mrs Hind said: "He was a hell of a nice bloke. Just a decent, nice, normal guy really. He was a great neighbour. We'd sometimes have small chats in the landing, just 'how are you?' sort of stuff.
"I'm Jewish and we had a conversation where I mentioned that. It didn't seem to phase him. I'm shocked."
Last night in London, Home Secretary Theresa May warned that "many hundreds" of people from Britain had travelled to Syria to fight against the Assad regime.
"They have ended up fighting for terrorist groups, often against other parts of the opposition rather than against the Syrian government," she said. "Some of them will present a real danger to us when they return to Britain."
Mrs May also said she thought women were among the 400 Britons who had gone to fight for Isis in Syria and Iraq.