Ga Bu isn't tempered. He is suffering from extreme dissociation due to the traumatic events he experienced. I.e. witnessing his parents' deaths and summoning a primal. There's a bit of ambiguity in the story around this, where Alphinaud briefly expresses concerns that the koboldling may be tempered but if that were the case they would have executed him ages ago.
Sin eaters are not related to tempering. Having your body's aetheric balance overwhelmed and corrupted by the pure aether energy of light is something entirely different.