The soul shows the true form of someone, even basic information like the person's sex is clear to those who can see the souls form – as in the case of Relius Clover easily determining Mai Hazuki's original gender within seconds of meeting her.
A strong soul is also capable of protecting its owner from the effects of the Boundary, as well as attracting the Blue, giving a special ability called Drive. If there is not enough magic element around, the use of Drive consumes a part of the carrier's soul.
Another type of special souls, are those possessed by the ones with the power to control the flow of life, as only those types of souls can be used as key-power source for the Kushinada's Lynchpin. Such soul had Celica A. Mercury.
After the life of the soul's owner ends, it returns to the Blue, where it is stored and preserved along with all the information it possesses.