Khalid Sheikh Mohammed said in a statement filed in Manhattan federal court late Sunday that Sulaiman Abu Ghaith served as an al-Qaida spokesman because he was "an eloquent, spellbinding speaker."
But Abu Gaith, Mohammed said, "was not a military man and had nothing to do with military operations."
Abu Gaith, who is a son-in-law of bin Laden, is charged with conspiring to kill Americans.
Prosecutors say Abu Ghaith was part of al-Qaida's deadly plot in his role as spokesman in fiery videos and as a motivational speaker at the group's training camps in Afghanistan.
Abu Ghaith's lawyers have said the Kuwait-born imam made inflammatory remarks but didn't conspire to carry out terrorism.
Defense lawyers are seeking to use testimony from Mohammed, who is in a detention facility in Guantanamo Bay, Cuba. They would need U.S. District Judge Lewis A. Kaplan's approval to introduce the information.
The defense has suggested Mohammed could help rebut the government's claim that Abu Ghaith must have known in advance of al-Qaida's so-called shoe bomb airplane plots, including Richard Reid's attempt to carry one out in December 2001.
The statement from Mohammed filed Sunday consisted of answers he gave to questions posed by Abu Ghaith's lawyers.
In the statement, Mohammed said he never spoke with Abu Ghaith about the shoe bomb operation and added, "those tasked with giving statements to the media do not necessarily know all the details of an operation and are sometimes even unaware of the very existence of the operation."
Prosecutors rested their case Friday in the trial of Abu Ghaith, the highest-level al-Qaida figure to be tried in the U.S. since the Sept. 11 attacks. The defense case is due to start Monday.