Board Thread:General Discussion/@comment-31371445-20170222233857/@comment-35381406-20181117021702

Determination cannot be the red SOUL's trait.

Firstly, as you said, Alphys extracted determination from each of the 6 non-red human SOULs, which wouldn't be possible if determination was the red SOUL trait.

Secondly, the game's manual states that the SAVE points are the manifestation of the human's determination, which wouldn't really make sense if determination was simply the human's SOUL trait.

Thirdly, if you get the red flag in the Ball Game twice, it'll say " RED - Try as you might, you continue to be yourself.", which, combined with other texts in the game such as "Despite everything, It's still you", implies that Frisk's SOUL trait is something along the lines of  being yourself.