I am thinking of another theory. If I'm looking into a couple of trilogies then it would be:
1/4 - The hero gets to use the True Rune featured in the game, which is mainly an offensive magic type.
Tir had the Soul Eater, while Lazlo had the Rune of Punishment.
2/5 - The hero does not get to fully use the featured True Rune, but the rune has two aspects - supportive and offensive. The hero gets to use the former, while his friend gets to use the latter.
Riou had the Bright Shield Rune, the supportive aspect of the True Rune of the Beginning, while Jowy had the Black Sword, which is the offensive aspect.
The Sun Rune could be said to have two aspects as well, though presented as helper runes, that is, the Sun rune is unstable without its helper runes. These are the Dawn Rune, the supportive type helper rune, and the Twilight Rune, the offensive type helper rune. The Prince of Falena has the Dawn Rune, while Lyon has the Twilight Rune. But of course we know that the Twilight Rune had been borne by someone else before her, particularly Alenia and Sialeeds.
3/6(?) - Multiple heroes, multiple True Runes. They prove that the Hero does not necessarily have to be the Tenkai star.
S3 featured the five True Elemental Runes: True Fire (borne by Hugo), True Water (Chris), True Lightning (Geddoe), True Earth (Sasarai), and True Wind (Luc)
From this theory, it could be deduced that Suikoden 6 may present multiple True Runes borne by multiple heroes. And, you guessed it, it is possible that we have a non-Tenkai hero.
It's not all about knowledge, but it helps.