The bindagent could very well be a person, but I don't think so. All uses of the "word" bindagent in contexts outside of this book appear to imply at it is authority or power that belongs to and must be assumed by another. There could be something controlling the Shadowblaze, but it would appear not to be a human. Based on some of the histories we have seen about the Spanish explorers we know they saw them on cliff walls. This implies that they could be:
A. The Rithmatic remnants of long lost, advanced civilization.
B. Wild chalklings that are "good".
C. The servants of a God.
First off, if they are chalklings they are far more advanced than any others we have seen created by humans so I am disinclined to believe that they of human creation.
I believe it is a combination of B and C. I believe that they are a natural occurrence within the system created by whatever supreme being controls this world in order to bestow Rithmatic power upon humans. The bindagent is obviously sentient otherwise it could not makes the wise decision, but I don't believe a human is controlling the system. The power that is the bindagent could very well be a being that attaches itself to the vessel very much like the forgotten attached to Harding, but the bond would be much stronger. Also the fact the book was written by humans, for humans doesn't mean they control it. We write about stars and we know what affects them, but we don't seriously impact them.