Ok for those who are new to Phantasy Star III or never heard of it, Gwyn is Adan's twin sister in the Adan storyline of the Third Generation of the game. With that said I can begin my rant.
During the coarse of Adan and Gwyn's travels through the Alisa III they come into contact with quite alot of things from their parents and grandparents. And they are the only ones who truly have ties to the Orakio/Laya war from the past. Aron coming from Lunes side of the family, where Adan and Gwyn come straight from the two sides, Orakio's descendants of Rhys and Nial, and from Laya's own sister.
During the story when the Nei weapons are aquired, Adan gets Orakio's Sword, and Gwyn has Laya's Bow. Two weapons that fought against Dark Force and ones that would have the most power against it, seeing as the sword was the item that sealed Dark Force, and the arrow was the one that pinned Dark Forces energy so the sword could seal it.
In the end Adan, Gwyn and the group successfully defeat Dark Force and they most likely go on to live happy lives for the remainder of their days. Now think about this.. If Adan was to happen to fall in the battle with Dark Force, the wielder of the sword, the weapon that sealed him would no longer have any power in the defeat of Dark Force.. But since Gwyn is Adan's twin, who comes from the same blood as Orakio and Laya, she would be able to take up the sword, and release its holy properties as well.
For my conclusion, I believe Gwyn is Adans strength, as he is her courage. They're both of the same blood, which makes them have the same destiny. They were both destined to save the Alisa III, both destined for great things, long before their birth, and most likely long after they are gone.