Serpent was indeed of immeasurable value to Sunny in his endeavor to master the next step of Shadow Dance. Given its ability to assume the form of any shadow resting in Sunny's soul — those equal or inferior to Serpent in Class and Rank, at least — the Legacy Shadow could be used to gain access to a vast repository of shapes for its master to study in profound detail.
As long as Sunny was using an incarnation to augment Serpent, he could experience complete unity with countless beings, from Dormant Beasts to Corrupted Terrors.
In fact… now that Sunny considered it, wasn't Serpent's [Formless] Ability more or less identical to what he was trying to accomplish? What he wanted to achieve was to learn how to mimic not only the outward shape of a being, but also their Attributes and mystical powers. And that was what Serpent had been able to do all along.
The final goal had been in front of Sunny all that time.
The problem, however…