This Roman does Posthumus a solid honor by letting him stay at his pad while he's in Rome. It turns out that Posthumus's dad knew Philario, so Philario's the guy Posthumus turns to when he arrives in the Italian capital. We don't see much of the guy, but we appreciate that he's around to knock some sense into Posthumus. Well, sort of.
When Iachimo tries to convince Posthumus that Imogen was quick to jump into bed with him, Philario steps in to defend her. When Posthumus freaks out at the sight of his wife's bracelet, Philario says, "Have patience, sir, / and take your ring again, 'tis not yet won" (2.4.143-144).
Ah, the voice of reason. Philario is a level-headed guy, and he tells Posthumus to take a chill pill about seeing the bracelet. He's right: Posthumus does believe Iachimo's claim way too easily, and Philario brings us back to reality—if only for a brief moment, because we're back in crazy land soon after.
When even Philario becomes convinced of Imogen's straying ways, we start to fear that all may be lost. Philario might not have a scene-stealer, but he sure is reasonable and logical. He could stand to teach Posthumus a thing or two.