Identify the speaker and explain Austen’s inference: “A young man too, like you, whose very countenance may vouch for your being amiable.”

The words come from chapter 16 of Pride and Prejudice and are spoken by Elizabeth Bennet in conversation with Mr. Wickham. The dastardly Wickham has been doing his best to build on Elizabeth's prejudice against Mr. Darcy and convince her that the man is even worse than she thinks. After Wickham spins a completely false yarn about how Darcy has taken against him for no good reason, Elizabeth exclaims

To treat in such a manner, the godson, the friend, the favorite of his father!

She could've gone on to say

A young man too, like you, whose very countenance may vouch for your being amiable

But instead contented herself with saying

And one, too, who had probably been his own companion from childhood, connected together, as I think you said, in the closest manner!

Elizabeth can well believe what Wickham's saying about Darcy. After all, the unflattering portrait he paints corresponds to her own disagreeable experience of the man. But she's also taken in by the habitual look of amiability on Wickham's face. This is why Wickham's such an expert liar; he looks like butter wouldn't melt in his mouth.
Elizabeth prides herself on being an expert judge of character, yet she shows herself as vulnerable to Wickham's good looks and charming manners as the flighty, foolish Lydia. She's judging Wickham on outward appearances, just like she did with Darcy. And in both cases she's reached completely the wrong conclusions.