Growth

Upgrading River of Vigor was a little painful. It was a powerful Soultrait, without a doubt, but it cost six times as many SoulStar Fragments as the average Soultrait to upgrade. Cosmic Stride was already a pain to upgrade, but River of Vigor Cost 1,500,000 SoulStar Fragments, pushing his upgrade cost of four Soultraits to 2.6 million. 

River of Vigor didn't evolve, but it changed quite a lot. The Soultrait could produce more healing serum after advancing to 7-Star, but that was only the tip of the iceberg. Michael could create different types of healing serum now as well. The information influx procured from upgrading his Soultrait showed him how to form three types of healing serum. The first was the ordinary silver serum and how to produce it more efficiently.