Final answer:
In Victor Hugo's 'Les Misérables', it is Inspector Javert who testifies that Champmathieu is, in fact, Jean Valjean.
Step-by-step explanation:
The question asks about a specific character witnessing against another in the context of a literary work, likely referring to 'Les Misérables'. In Victor Hugo's novel, several characters wrongly testify against Champmathieu, believing him to be Jean Valjean. However, the key moment comes when Inspector Javert testifies with certainty that Champmathieu is Jean Valjean due to his own past encounters with Valjean and belief in his criminal nature, leading to a dramatic court scene where the real Jean Valjean ultimately reveals his identity to save the innocent Champmathieu.