First, off the Radiant have no qualms (to my knowledge) about enslaving spren, the Oathgates are powered by spren and in one of Dalinar visions we see a Radiant use a heliodor fabrial. I think that the spren is Re-Shephir, note how she was trapped in a pile of gemstones and was the source of the tendrils of wrongness that Shallan feels in the tower, the ones that disappear when Re-Shephir runs away.