How did Axel know about Leons feelings for Riku?
Leon’s not as mysterious as he makes himself out to be. Leon’s feelings for Riku are actually very obvious, especially given their past, and how often Axel has run into them together. All of that will be made clear in LNOL, the prequel. Plenty of other people suspect it, too, like Sephiroth and some of Riku’s friends. It will be pretty obvious in LNOL to everyone except for Riku; and the reason it’s not obvious in 180 is that Sora doesn’t really know their pasts.