I don’t think it was simple as him trusting her completely. He “needed” her as means to an end, but slowly got snared by darkness. The first game sets up the theme of the darkness slowly controlling you, as with Clayton and many Disney villains, so I think Riku trusted her results but knew deep down she was dangerous. Also despite his facade, Riku had moments of weakness and felt insecure. In those moments I don’t doubt that he ignored the warnings and jumped aboard. Personally I see desperation in Kingdom Hearts 1 Riku. It’s so subtle, but it becomes plainly obvious when he loses to Sora, because despite being the rational one, Riku is also very emotional.