Billy Windsor, at a glance, looks like a convenient Mike substitute. And in a lot of ways he is. But in terms of how Psmith relates to him, he fills a very different role, one unlike other interactions Psmith typically has.
Psmith meets Billy after cleverly getting him out of trouble in a restaurant and from there it escalates to Psmith’s planning to completely revamp Cosy Moments with him. This is a typical start for Psmith; he’s always the idea person in friendships--he can’t help himself. But once he sets the idea in motion, he seems content to contribute writing and let Billy handle most of the management. They work together, but as sub-editor, Psmith is in a subordinate role.
This isn’t usual for him. We’ve seen especially in Psmith in the City that he tends to take charge, to assume power, to make all the decisions. As amiable as he generally is with Mike, once his mind is made up he will listen to no objections, and he’s not above resorting to manipulation. He’s used to being the most influential person in the room.
But with Billy, he’s interacting with someone who is technically his boss/supervisor, several years older than him, and far more experienced in the ways of America/New York and life in general. Billy is constantly explaining things to a confused or uninformed Psmith. He’s protective of this rather naive, sheltered English kid (...Psmith is twenty, but still. awfully young) and tries to keep him from making careless mistakes that could get him killed. He’s quick to take situations in hand--especially in a fight. And Psmith...defers to him. Never even tries to manipulate him! There’s never any power struggle between them. Psmith’s original objective in taking up journalism is his own amusement, and Billy’s leadership role frees him to devote his energies to observations on life. It’s maybe the closest he’ll ever get to having a benevolent older brother figure.
That is, until the switch. At some point after the hat-shooting, Psmith leaves off being out of his depth in New York and unlocks his action hero mode. He fights alongside Billy on the tenement roof. And suddenly it’s Billy who is making careless mistakes from which Psmith must rescue him--not noticing that they are followed and his pocket picked after they learn Waring’s identity, and getting sent to prison (where he will spend the rest of the novel--we don’t see him in person after Psmith meets him at Jefferson Market). Which in turn leads to Psmith making the gravest error of all by believing a forged note “from” Billy and getting a cab to reach him--where he will be held at gunpoint. These situations are closer to the Mike-and-Psmith dynamic (rescuing the hapless friend from the mess he’s gotten himself into). But not quite.
Because Psmith and Billy’s dynamic is built on working on a project together. They’re business partners--amiable and friendly--but not too close. Billy has none of Mike’s gift for reading Psmith, and Psmith on some level seems to regard Billy’s Americanness as a bit of a joke. They’re both aware that after ten weeks or so, it’s all over, so there’s no intention of bonding on too personal a level. Nothing wrong with this, by the way. This is an observation, not a judgment. A longer-term partnership--say, in some alternate universe in which Psmith opts to pursue journalism in New York permanently instead of returning to Cambridge--might not have worked. Billy’s hot temper and tendency toward violent solutions might eventually be too much for the detached, even-tempered Psmith. And it would require a saint to put up with Psmith for an extended period.
Mike and Psmith’s dynamic, on the other hand, is based in being companions in misfortune. Permanent company for each other. Emotional support comrades or something. They do things together, but the schemes and adventures--at this point--are a result of their bond rather than the reason for it. Psmith talks business and makes flippant commentary to Billy, but it’s Mike to whom he gives his “these are the real Cosy Moments” speech at the end. When Psmith and Billy are together, they’re in the office or fending off trouble somewhere around town; the most leisurely moments we get of them are in restaurants. But Mike and Psmith can hang out in Psmith’s Cambridge rooms at the end and fall asleep mid-conversation with complete casualness.
And both of these are important to character development. We’ve seen Psmith get close to his single solitary good friend; his cultivating a sincerely friendly and nonmanipulative relationship with a more casual friend is another step toward maturity.
If Psmith’s friendship with Billy is the frenzied excitement of keeping busy on a vacation, with Mike it’s coming home at the end of a long day. Both needful, but one more sustainable in the long term than the other.