Stefan Millar told a jury that he did not kill the 16 year-old in the Western Isles.
He also claimed that he had never been to the derelict building where Mr Aitchison’s body was discovered.
Millar gave evidence on his own behalf at the High Court in Glasgow.
He and co-accused Johnathan MacKinnon - both 22 – are charged with murdering Mr Aitchison in Steinish on the Isle of Lewis on November 23 2011.
Mr Millar told the court how he had got to know Mr Aitchison while they worked separately on fishing boats.
They soon started socialising together and – on November 22 2011 – they went to MacKinnon’s home in Stornoway.
Millar’s QC Frances McMenamin asked: “What was the atmosphere like in the room?”
He replied: “Happy – having fun.”
The court heard the trio left the house in the early hours and Millar recalled Mr Aitchison “messing about” - including sitting in puddles.
Millar said he then remembered a bottle of aftershave falling out of Mr Aitchison’s pocket before MacKinnon allegedly punched the teenager.
Millar denied getting involved in the altercation.
Miss McMenamin asked: “Did you ever lay a finger in anger on Liam Aitchison?”
He replied: “No.”
Millar claimed they later parted company with Mr Aitchison. Millar said he believed the teenager was going to a friend’s house.
He denied there was any “fall out” before they went their separate ways.
Miss McMenamin also asked: “Did you murder Liam Aitchison?”
The QC: “Did you play any part in his murder?”
The accused: “Not at all.”
The court previously heard claims that Millar “bragged” about being involved in the killing to a cell mate in Inverness prison.
But, Millar denied that he said such a thing.
Millar was also quizzed about the property where Mr Aitchison’s partially-clothed remains were found.
Miss McMenamin asked: “Your position is that you have never been in the house?”
Millar: “Never in my life.”
MacKinnon and Millar deny murder.
It is claimed Mr Aitchison was struck on the head and body with a knife and bottle as well having his head stamped upon.
The trial, before Judge Lord Kinclaven, continues.