Board Thread:Questions and Answers/@comment-31155663-20170228142325/@comment-35248429-20191017003123

87.110.155.250 wrote: The things is, in season 3 when Asuna and that lady doctor went to ocean turtle the dude said that now there are 3 members there but he couldn't reviel the 3rd member and I think it could be Kayaba's AI.

Sorry for grammar mistakes and not knowing few ppls names :P The third person is Kirito

Pretty sure that's directly stated in the episode.