Board Thread:General Discussion/@comment-26127761-20160331033430/@comment-32182236-20180108160909

PancryPancreas5 wrote: Even if monster and human combine souls, it's unknown if they can go back. Also, another plot hole is what happened to Chara's soul after Asriel died, and how can they control Frisk/player... It shattered like Frisk's does. Same color, and explains why their SOUL wasn't captured.

Just like monsters, humans have essence. Chara's essence was in the Ruins where we first fall, and we awaken them with determination. Some of the essence probably latched onto us... As we give them LV, they gain more power...