Foil
Character Role Analysis
Chad
Strether heads to Paris thinking that he'll find Chad Newsome living a life of total sin, sleeping with random women and drinking through the night. What Strether doesn't expect to discover, though, is that Chad has somehow turned into a perfect gentleman since leaving The States.
Once Strether wraps his head around this fact, he starts to become obsessed with Chad's awesome Paris life. In fact, all he can see in Chad at first is all of the opportunities for adventure that he (Strether) wasted in his own youth.
This is pretty much the definition of what a foil does: shows another version of a character with slightly different traits and choices, but some similar core features. Kind of like the same guy but in a parallel universe.
Anyway, in this case, Chad is like a mirror, in which Strether sees the reflection of all the things he wishes he did with his own youth. Unfortunately, those days are past, and Strether needs to suck it up and start living life in the here and now. Chad even becomes a sort of hero for Strether, because that's how bitterly Strether wishes he were young again like Chad.
Strether shouldn't be so hard on himself, though. He often loses sight of the fact that Chad's life is only made possible by his inheritance from his rich grandfather, while Strether grew up with much less money. The comparison isn't as easy as Strether seems to think it is. If only he could read this now and be comforted.