Seok Dong-hyeon, a lawyer and longtime friend of Yoon, also told reporters that multiple - seemingly overlapping - investigations involving Yoon should be streamlined.
Yoon, who is an ex-prosecutor, shocked the nation on December 3 when he declared martial law in a late-night televised address, before backing down hours later after outraged MPs rejected his decree.
"He (Yoon) has already apologised for surprising and shocking the public, and he remains apologetic for that ... but he has a clear and confident position on the issues that are in dispute and must be addressed," Seok told a briefing on Thursday, adding Yoon had never even considered insurrection.
Thousands of South Koreans have taken to the streets against Yoon since December 3, and Yoon faces a Constitutional Court trial on whether to remove him from office or restore his presidential powers after parliament last Saturday voted to impeach him.
President Yoon Suk-yeol he declared martial law in a late-night televised address on December 3. (AP PHOTO)
Yoon also faces investigations into whether the martial law declaration constituted insurrection, one of the few charges for which a South Korean president does not have immunity from.
"What kind of insurrection follows parliament's order to stop ... and quits after two to three hours?" said Seok, who was wearing a souvenir watch from Yoon's presidential office.
Asked whether the martial law decree breached the constitution, Seok said Yoon viewed the situation of an overbearing opposition party controlling parliament, cutting the government budget and impeaching government officials as an "emergency state" and the basis to invoke martial order.
Yoon was staying at his residence with his duties suspended due to parliament's impeachment, the lawyer said.
While Yoon had apologised for rattling the nation with his martial law declaration, Seok also called for public understanding over the president's "pain" dealing with the opposition's excessive power.
It remained unclear if or when Yoon might comply with the investigations or respond to attempts to summon him.
Thousands of South Koreans have protested against Presdient Yoon Suk-yeol since December 3. (AP PHOTO)
He failed to appear on Wednesday before a corruption watchdog for questioning over his martial law attempt, and also ignored a summons on Sunday from the South Korean prosecutor's office.
The Constitutional Court has ordered Yoon to submit his martial law decree, as well as the minutes of the cabinet meetings held before and after the declaration.
South Korea's acting president, Han Duck-soo, spoke with Japanese Prime Minister Shigeru Ishiba by phone on Thursday.
They agreed to continue three-way security co-operation with Washington while facing growing North Korea-Russia military ties as well as Pyongyang's nuclear threat, Han's office said.
The investigations into the martial law declaration have also embroiled a number of officials and security personnel, the latest being former army intelligence commander Noh Sang-won, who was arrested late on Wednesday.
with EFE
