At the end of the day, the purpose of souls and the mechanics of transactions concerning them is going to down to what the writers like Savin and The Observer decide, end of story.
Yes, there is a power to a soul, and someone who might be able to acquire more souls might put them to a greater use, albeit with some caveats, such as the Kitsune collective soul. And yes, one such as Keros might have the ability to hold onto the soul and create a simulacrum to deceive others. But it also seems that just selling your soul doesn't mean that much, at least from a power position. Yes, the Champion might pledge their souls to other beings like Keros, but in doing so don't lose much. They still retain their empathy and that spark of compassion we might call humanity. From a power perspective, that spark is still dangerous, because it makes the Champion unpredictable.
In other fiction, there are two very broad types of people who sell their soul - Those who do it for the wrong reasons, like greed, power or any number of selfish reasons. And we're taught those who do so find it fleeting, a poor exchange for something so valuable.
And then you have the ones who sell their souls for the right reason, to save someone they love, might be a parent, spouse, child or friend. Whatever it might be, it is still a (near) selfless act, they've weighed the cost of their own soul against someone else and deemed it a worthy cost.
Now, depending on who might be looking for that soul, that's a dangerous proposition. If you're a devil and specialise in tormenting souls, well its hard to do that when the shining beacon of decency is giving off hope in hell.
Depending on how you interpret Keros' offer and the Champion's reason for taking it (power, integration or protection their kitsune family), there is still a definite power imbalance. Keros needs the Champion's soul a lot more than they need to be a Keros' personal champion. And that puts the Champion of Hawkethorne in an uneviable position of power.