Board Thread:General Discussion/@comment-38742181-20190308020842

Theory: The red SOUL represents freedom.

Reasoning: First, freedom can be considered a trait in that you can be a free soul just like you can be a brave soul. Second, Frisk and Chara both had the red SOUL, but had completely different personallities. This means that they were free to think and act as they wanted, and that also explains why there are multiple endings. Third, you are able to move freely on the "bullet board" and change "heart modes" at will. It can't be determination since that is what makes a SOUL run, and it can't be nothing since the soul has color (not white like the monsters' SOULs).

What are your guys's thoughts? 