Vital Gu

The Vital Gu is the first Gu refined after the aperture is awoken while, the ones that follow are known as "Ability Gu".

The Vital Gu is the Gu bound to the life of its Gu Master, and as such unless the Gu Master dies the vital Gu will be alive.

Unless certain circumstances the first Gu worm will have the same rank as the Gu Master - a Rank 1 Gu Master will have a Rank 1 Vital Gu.

If a Gu Master somehow loses literally all of his Gu Worms or if the Vital Gu is taken by someone else than if the Gu Master doesn't die then there's a chance to seek a replacement.

Value
When a Gu Master refines the Gu Worms of the next rank via fusion there is a big chance for the fusion to fail resulting in either the loss of all of the Gu Worms at its worst or damaged Gu Worms at its best.

This defect can be curved by using the Vital Gu since no matter the times a fusion might fail the Vital Gu will always survive. This is also the reason why the first Gu to be fusion refined is the Vital Gu.

The Vital Gu is also the the point that decides your road, though that doesn't mean it is mandatory to be the case.