Mr Sikorsky's legal team will argue Larissa died as a result of "misadventure" — not from drugs.
Detectives are still waiting on toxicology reports, which will tell them if Larissa had drugs or alcohol in her system when she died. They are also waiting on the autopsy results, detailing how and when she died.
News.com.au understands a post-mortem was completed on Friday.
"I absolutely believe he's innocent, and I've been doing this a long time," Mr Ryan told the ABC.
"I think if the police do their job they'll discover he didn't commit murder.
"If they dot their i's and cross their t's they will discover that he didn't do anything and rather she may well have passed through misadventure."
Police launched Operation Quebec Jagger on June 27 after Larissa's body was found inside the barrel, sitting on the back of a black ute Mr Sikorksy had allegedly fled a Logan City home in.
On June 28, Queensland Police made an urgent appeal for information, warning the public Mr Sikorksy could be armed and asking them to immediately call triple-0 if they saw the 34-year-old.
On the afternoon of June 29, Mr Sikorksy was located in a Sunshine Coast apartment and, after a lengthy standoff with police, surrendered peacefully more than 27 hours after officers had surrounded the apartment.
A 40-year-old Palmwoods woman and a 38-year-old Alexandra Hills man, who were allegedly inside the apartment when the siege took place, have both been charged with one count of accessory after the fact to murder with the pair also appearing in Maroochydore Magistrates Court today.
It's alleged the 40-year-old woman let Mr Sikorksy inside the apartment.
Larissa was reported missing on June 26, almost two weeks after her last conversation with family and friends.
Police investigations into Larissa's disappearance led them to a Buccan property, south of Brisbane, early last week where Mr Sikorsky allegedly fled in the black ute.
It's alleged he arrived at a nearby gated complex in the Gold Coast suburb of Stapylton where a female resident claimed he demanded to be let into her housing estate.
The woman said it was at that moment she saw a "dead arm" sticking out of the blue barrel.
"There was a blue barrel with, all I've seen is an arm flapping out, it was a little bit scary," she told reporters.
"There was just the one barrel, it wasn't massive either, it was only small. I just seen (sic) an arm, a dead arm."
Larissa's sister Deanna mourned the 16-year-old's death in a heartbreaking Facebook post late last week.
"With a broken heart shattered into more than a million pieces our family is utterly destroyed to inform you all that Larissa has been taken from us," she wrote.
"Our beautiful girl has been reunited with our beloved wife and mother. Fly high our darling angel, until we see you again Rissy."
