Titans and their Spawn represent Fear, your mind's instinct to not do something, wanting to avoid the possible consequences of that action.
All lightners possess Determination.[1] Combining this with the SOUL containing immense power over light, as well as possessing Determination, and having a 'certain power', heavily implied to be the same as Determination, that seals fountains of darkness and manifests in the form of sparks of light, we can assume that light and Determination are the same, or at least heavily intertwined.
Thus, this means that Titans, the deepest dark, created by Fear, is opposed by light, intertwined with the power of Determination, the ability to continue on no matter the circumstance, even if the consequences are horrible (see: Weird Route).
This means that the playable SOUL, made of light and Determination and holding immense power over them, and being the explicit counter to a Titan, is the opposite of a Titan. Titans and the SOUL are opposites in other ways as well. Titans are completely mindless, while the SOUL is nothing but a receptacle for the will of the player.
What I find interesting is that Determination is what is used to create Dark Fountains, which create Titans, which means that the Dark Fountain creation process inverts Determination somehow.