Character Analysis
This isn't Billy's story.
Neither of the Kramers in the film's title are him. Nope, Kramer vs. Kramer is the rare movie about a kid's parents getting divorced where the story isn't really concerned with the kid. Billy may be caught in the middle, but the story's about Ted and Joanna, not about Billy.
The Pint-Sized Professor
Okay, so it is about him, in that both of his parents love him a bunch and want him to live with them, but Kramer vs. Kramer is his parents' story. It's the story of his mom leaving to find herself so she can be a better mother. It's the story of his dad being forced to grow up and stop being so self-centered.
To that end, Billy's primary role in the film is to teach Ted how to be a dad. On the practical end of things, he has to show Ted stuff like where they keep the bowls and pans in the kitchen; he has to share his morning routine with Ted, and show him how they get to school. His life's been upended, and he just wants the stability of knowing where his bed and toys are going to be
Big picture-wise, that means Billy teaches Ted what it's like to care more about another person than you do about yourself. When Ted's late, Billy lets him know it:
TED: I'm not that late, Billy. I'm only 20 minutes.
BILLY: You wanna make a bet?
TED: Yeah.
BILLY: All the other mothers were there before you.
Did you catch that Billy lumps Ted in with all the mothers? To Billy, moms and dads are fundamentally the same. For Billy, being a parent isn't dependent on gender; it's dependent on being patient, being protective, being kind, and perhaps more than anything, simply being there. On time.
Billy's a pretty unflappable kid (just ask Phyllis), but he needs stability. That's why two of his biggest concerns when Ted breaks the news to him that he's going to be moving in with Mom and sleeping over at Dad's every other weekend are where his toys and his bed are going to be.
You Can Count on Me
Billy's world may be small, but he needs certainty within it. Faced with prospect of his routine changing once again, as well as the realization that he won't see Ted very often anymore, Billy's justifiably upset:
BILLY: You're not gonna kiss me goodnight anymore, are you, Dad?
TED: No, I won't be able to do that, but, you know, I get to visit. It's gonna be okay. Really.
BILLY: (crying) If I don't like it, can I come home?
That scene is really hard to watch.
Billy may be able to roll with the changes exceptionally well for a six-year-old kid, but he's still very much a kid, and being a kid means being full of contradictions. Odds are, you don't remember what it was like to be six-years-old, but you do remember what it was like to be 16 years-old. Maybe you're 16 right now. Maybe you will be next year, and, in that case, Happy Early Sweet Sixteen to you. Sorry we didn't get you a card.
Where were we? Oh, yeah: being a kid is confusing whether you're six or 16 (or, in some cases, 46). You're irrational, and you don't know how to weigh the importance of things accurately. Everything seems like THE MOST IMPORTANT THING EVER. That's why Billy, and every other kid, needs stability; they need consistency to be the backdrop for their self-created craziness.
In the end, that's what Billy teaches Ted to provide: a stable, loving home where he has the freedom to be a completely illogical first-grader who wants to eat a gallon of ice cream for supper one minute…and a tiny, blonde Yoda who mentors his dad's metamorphosis from selfish jerk to doting dad the next.
Justin Henry, who hadn't acted before being in this film, got universally rave reviews (and an Oscar nom) for his no-nonsense, emotionally honest portrayal of Billy. It can't be easy directing a young kid (Henry was seven years old during filming), or acting with one for that matter. But Henry, reviewers agreed, made the scenes with Hoffman totally authentic and believable.