You can't be serious about monotheism being a default assumption for characters in the media. At best, characters are assumed to be wistfully agnostic, occasionally "spiritual but not religious." If Avengers III has a scene of Iron Man getting a phone call in church telling him to suit up and get on the job, everyone will find that jarring because it's never been established or even hinted ANYWHERE that Tony Stark would even go to church in the first place. Furthermore, can you imagine the uproar if it Bruce Banner, Hawkeye, and Black Widow were all with him at church? You couldn't just say, "Of course they're all Christians. It's the default assumption until stated otherwise, right?" because everyone knows the opposite is true in modern media.
The U.S. has one of the largest church-going populations in the western world; I don't see why it's in any way non-serious to assume that (in the absence of evidence to the contrary) a great many characters in film are Christian, Jewish, or Muslim. Certainly not all of them are, but it would be fairly safe to assume a great many likely are.
And let's not forget that, in general, a character's religion is going to be of far less importance to a story than their sexuality will be. Generally speaking, who a character is romantically and/or sexually attracted to is going to play a part in most stories, because most stories include some kind of sex and/or romance. Meanwhile, religion in film really only comes up if the film is a horror film, or if the character is, or pretends to be, devout.
Also, if we're talking about characters in film being religious, we should probably concern ourselves with how genuine they are. The most famous church-going film character I can think of is Michael Corleone, who certainly should not be seen as being religious if we're considering adherence to a professed faith as part of depicting a monotheist.
Now, to your question about monotheists in media, Captain America has already been mentioned. Peggy Carter also likely was, since they were holding her funeral in a Christian church in Civil War. In Wonder Woman, the town they liberated had what appeared to be a Christian church, so many of those townsfolk were probably Christian. I would assume that Noah, from the 2014 film was a monotheist, but then I don't go to see many movies, and that wasn't one I saw. Superman goes to a Christian church to help him make a difficult decision in Man of Steel and actually talks briefly to a priest, which likely means that Ma and Pa Kent were both Christian. Batman's parents appear to be having a Christian burial in the early scenes of BvS. Daredevil is a notorious catholic. If the 2013 remake mirrors the old film, Carrie and her mother would be Christian (albeit non-conventional ones).
If you can even spot a character who kisses a menorah or wears a crucifix or goes to church, I'll settle for that too. Just any of the things that millions of Americans do on a regular basis, and Hollywood moviemakers apparently never do.
Now, now, let's not make assumptions about people we've never met. I'm Catholic and attend mass every Sunday (at least, I did before my father got so ill I had to quit work and care for him 24/7; I can't leave him alone long enough to go to mass anymore.) but you won't find a lot of monotheist characters in the fiction I write. You will find some. But, if I'm making an honest evaluation, I have one work of modern supernatural fiction that has two Japanese Catholics in it and one work of sci-fi fiction that has a Christian of non-specified denomination, but the rest of my work typically features polytheistic characters in a fantasy environment, or largely non-religious characters in a sci-fi environment.