Either the Federal army, or the local people. I would think that most of the deaths occurred near Nashville on December 16th. The Confederate hospitals at Franklin were overrun by the Federal pursuit the next day. Most of the contact from Nashville to the Tennessee River was rear guard skirmishing. Forrest's Cavalry was involved in covering the retreat below Franklin.
Are you looking for a particular missing soldier? Many soldiers ended up "missing" from the Compiled Military Service Records after the fighting at Nashville. Many escaped across the Tennessee River and went to hospitals in Mississippi or Alabama, or simply went home. You would have to find a post-war record of them to know that they survived.