Ernest is a character from episode 8, "Once Upon A Time", of Devil May Cry: The Animated Series.
Ernest finds Dante and immediately recognizes him as an old friend of his named "Tony." He invites Dante back to his town. Dante repeatedly insists that he is not the same Tony, though at one point he does concede that he used to use that name as an alias. Nonetheless, Dante goes with him, and Ernest explains the history of his town and his relationship with "Tony". Apparently, when he and "Tony" were children, a massive catastrophe struck the town, and the townsfolk blamed Tony and his mother. Ernest couldn't believe that Tony was the cause of the disaster, and spent the rest of his life until now trying to prove it.
He thinks he finally has the proof he needs, and tries to show Dante. Unfortunately, Ernest ends up summoning a powerful demon, which shouts Dante's name as it emerges from the portal. Dante fights and kills it, but this causes Ernest to change his mind. He now not only believes that Dante is Tony, but that he was also the cause of the disaster. Ernest yells Dante to leave town and never come back (threatening to kill him if he will return), which Dante does.
Ernest is derived from the German word Ernst, which translates to "serious," which seems to hint at his serious nature regarding the demon attack and his reaction to discovering Dante's true nature after he summoned the demon.