Saïx and Xemnas met after the former was recruited into Organization XIII. Xemnas actually trusted Saïx so much that he made him his second in command and put him in charge of handing out missions. However, Saïx and Axel initially had plans to take over the organization for themselves. But when Axel begins to spend more time with Roxas and Xion, Saïx grows apart from him and becomes more loyal to Xemnas. After they were both destroyed, Saïx reappeared as Isa but still remained loyal to Xehanort. He is to become a vessel for him and does not seem to mind.
XemSai is a very popular ship in the Kingdom Hearts series and is possibly the most popular ship involving both Xemnas and Saïx. While some fans would prefer AkuSai, they feel that Roxas has gotten in the way of this ship. Many fans thought that Saïx was loyal to Xemnas from the start and that Xemnas trusted Saïx as well. When it was revealed that Saïx is still loyal to Xehanort, some fans were turned away from this ship while others began to support it more.