Prior bad act character evidence is generally not admissible (person did something bad in their past so it is likely they did something bad now = propensity evidence).
However, under 404(b) there are exceptions to this rule: evidence of prior acts to show motive, identity, plan, etc.
But 404(b) can be used for witnesses other than the defendant:
Hammer v. State, 296 S.W.3d 555, 565–66 (Tex. Crim. App. 2009)
*Texas and SC have very similar rules of evidence*