The early scene on the boat between Rikku and Tidus certainly made me wonder what might have been had he not been (SPOILER)whisked away across the ocean. They certainly have the most similar personalities. The game seems to toy with us that Tidus could gain affection from any of the three female leads, but in the end it just affects some cutscenes and limit breaks.

My most plausible theory is that the GF Eden if FFVIII is actually a mechanical version of a Garden, hence 'Garden of Eden'. She (yes Eden is female) has the same basic ring structure in the upper portions.

Less plausible is that the 'In the Garden Sleeps a Messenger' refers to Diablos sincer his summon is called 'Dark Messenger' and he 'sleeps' in a lamp at Balamb. But it could be just a radom puzzle.