DnD 5e - Character Concept
The first step in building a character is to come up with a general idea of what you want to create. It's totally fine to stick to classic fantasy tropes (the noble knight, the wise old wizard, etc.), but you're free to make your character whoever you want them to be.
Keep in mind that your character will usually start at level 1, so their heroics prior to the start of the campaign are typically minor. Many players new to Dungeons and Dragons look at their favorite characters from fiction at the end of their stories, when the character is experienced and powerful, and wonder why their 1st-level character doesn't match that expectation. Remember: your character is probably new to adventuring. However, at level 1 they're already skilled and powerful compared to the average person. They might have a local reputation for their abilities, but their renown likely doesn't extend beyond their neighborhood.
Borrowing Concepts From Fiction
If you don't already have a character concept in mind, it's easy to borrow a character concept from a piece of fiction which you enjoy. Emulating characteristics of a character that you like is a great basis for a character, and adding your own spin to that concept can create rewarding, interesting characters.
Below I'll provide some usable examples from a few pieces of fiction. Each of these examples are open to interpretation, and your own interpretation might vary wildly from my own. These are intended merely as a starting point to help fuel your imagination. I'm also not an expert on these works of fiction, so devoted fans will probably be vigorously offended by my shallow interpretations of the characters. To those people: I'm sorry. Please feel free to email me about it.
I'm going to mention a few terms below which haven't come up in previous sections of this guide. We'll cover those later in this section of the guide.
Lord of the Rings
Lord of the Rings did a lot to establish contemporary fantasy, and its themes and characters are well-established and easily understood.
Aragorn goes through a few changes throughout the course of Lord of the Rings, and honestly he's a bit of a "Mary Sue". He seems to always have a solution to every problem, he's good at basically everything he does, and at the end of the story he gets to be the king. He can't do magic, sure, but Lord of the Rings has much less accessible magic than Dungeons and Dragons.
Aragorn could be interpreted as either a human or a half-elf (he has some elven blood, so he lives much longer than humans), and either a fighter or a ranger. He's almost certainly Lawful-Good.
Boromir is short-lived, but he's still important and he's a good archetype. He's more pragmatic than the rest of the fellowship, and he's more willing to accept the risk of using The One Ring if there's a chance that it could turn the tide of the war his people have been fighting since long before he was born.
Boromir is a human fighter. He's not fancy, he doesn't know fancy tricks. He fights, and he's really good at it. He's likely Lawful-Neutral because he cares more about duty than he about right and wrong.
Frodo is surprisingly difficult to pin down. Early in the trilogy he's a naive youngster who wants to go on an exciting adventure. As the series progresses he steps up and accepts the responsibility to carry The One Ring, but aside from carrying the ring and suffering the associate consequences, he really doesn't do a whole lot. He's basically an escort quest NPC.
The best I can guess is that Frodo would be a rogue. Definitely Good-aligned, but I can't guess where he would fall on the Law-Chaos axis.
The classic elderly wizard, Gandalf is wise, knowledgeable, and posesses fantastic magic abilities that others can scarcely comprehend.
While Gandalf can't be perfectly interpreted in Dungeons and Dragons because of his unique origins, you can roughly approximate Gandalf as a Lawful-Good human wizard, probably with the Sage background.
Gimli is, in many ways, the dwarfiest dwarf that ever did dwarf. He doesn't like elves (typically not a thing in modern Dungeons and Dragons settings), he likes axes, he likes mining, and he fights orcs. Truly a paragon of dwarfs.
Gimli is a dwarf fighter. I would say probably a mountain dwarf, but there's room for interpretation and the philosophical differences between hill dwarfs and mountain dwarfs basically amount to "can I walk to a place with sunlight and be home in time for dinner". Gimli is likely either lawful-neutral or lawful-good.
Legolas is the first elf we see for an extended period in Lord of the Rings, so he does a lot to establish exactly what elves are and how they do things in Middle Earth. As such, he's a bit of a "generic elf", but he's still a fine example.
Legolas is either a wood elf fighter or a wood elf ranger. Ranger makes a lot of sense because his people live in the wilderness and he switches between archery and two-weapon fighting, which are the go-to options for rangers ind Dungeons and Dragons. He's probably neutral-good or chaotic-good, but I honestly don't know.
Merry / Pippin
Merry and Pippin are stand-ins for the reader. They're there to be the reader's eyes and ears, and for the other characters to explain to them what's going on. They're in the story mostly by accident, and they're carried between major events repeatedly with little effort to do so on their own. Still, they step up a few times when the stakes are really high, and they come home from their journey as strong, experienced adventurers.
Merry and Pippin probably have identical builds. They probably started as chaotic-neutral rogues, then gradually shifted to chaotic-good and started taking fighter levels.
Sam is the best friend that anyone could hope for, and I think he's one of the more novely members of the Fellowship of the Ring. He's the classic farmboy-turned-hero, which is a classic trope. There's a girl he's pining after back home, and as much as he misses the comforts of home he knows he's got a job to do and that Frodo needs him, and he does't balk at that responsibility.
Same is probably etiher lawful-good or neutral-good, and he's either a fighter, a rogue, or a combination of the two.
Marvel Cinematic Universe
Super heroes don't map to Dungeons and Dragons as well as the Fellowship of the Ring, but there's still a lot of inspiration there, and since the Marvel Cinematic Universe films are still running, it's a great contemporary example.
Characters are listed alphabetically by their superhero name.
Ant-man is a rogue (thief) with the Criminal background. He has historically been fine with breaking the law, but he's still mostly a good person, so chaotic-good seems like a reasonable alignment.
Definitely rogue (assassin), but it's hard to find a published background that matches Black Widow's backstory. Something, something, budapest. Alignment is difficult. I think Black Widow's motives go a bit beyond the urge to do the right thing, and she has some serious emotional baggage that make her difficult to analyze. I would probably say true neutral, but that's basically a guess.
I can't think of a better example of a fighter, but Cap could be a champion or a battlemaster depending on how you want to interpret him. The Shield Master feat seems like a good addition. Captain America is all about doing the right thing and living by a strict code of right and wrong, so he's as lawful-good as it gets.
If such a thing existed in the Marvel C.U., I think Captain America would be a paladin. Believing in something so hard that it gives you magic powers seems like something Steve Rogers would do.
Captain Marvel is hard to pin down because she's so powerful, but I think the best we can guess is a chaotic-good sorcerer of some sort. She essentially runs on magic that's central to her physical being, and learning to assert her sense of self allowed her to tap into her full power, which is basically how sorcerers cast spells.
Falcon is a lawful-good human fighter, and the Dungeon Master gave him a bunch of cool magic items because his stats aren't good enough to compete with Captain America's after the DM gave him that potion that permanently increased all of his physical ability scores. The Soldier background seems like an obvious choice.
Hawkeye is either a rogue or a fighter, or a multiclassed rogue-fighter. Based on how weak he is compared to most of the other avengers, he seems like the character who's too low-level to be in the party, but the DM keeps putting him in right place to make him crucial when the stronger characters are elsewhere. Alignment is hard, but I would say probably neutral-good pre-Thanos, then arguably chaotic-evil post-Thanos.
Barbarian. The Hulk's whole thing is getting angry and smashing stuff. The Hulk is probably chaotic-neutral. Dr. Banner is probably neutral-good, but I don't think there is a published class that neatly covers a scientist who can't fight. You could used artificer or wizard, but neither work perfectly.
Tony Stark is a Mary Sue if ever I've seen one. Incomparably smart, charming, rich, and able to invent his way out of basically any problem. His alignment fluctuates as the character grows, starting from chaotic-neutral, and probably ending somewhere close to lawful-good. Class-wide, artificer or wizard seem the most likely. The Noble background seems like a close fit.
The Scarlet Witch is a sorcerer. Her magic is innate rather than learned or granted. Her alignment is hard to guess, but she's probably close to chaotic good. I don't think there's a published background that lines up with her origins among Hydra.
Spider man doesn't map to a Dungeons and Dragons character at all. Peter Parker is almost certainly lawful-good, but there isn't a way to mirror Spider Man's powers.
Thematically, a cleric (tempest) makes the most sense, but fighter (eldritch knight) might also work. Despite his carousing and other shenanigans, Thor is probably lawful-good.
Vision draws his power from something outside himself, and his primary way to fight stuff is to shoot it with a laser, so warlock seems like a good fit. Lawful-good seems likely, but we haven't seen Vision on-screen enough to really know his personality.
Similar to Falcon, War Machine is a human fighter with cool items. Soldier seems like an obvious choice for background, and lawful-neutral seems like a good fit for Rhodey's duty-driven personality.
Character Concepts to Avoid
Not all character concepts are good. Some concepts are so bad or so problematic that many groups outright prohibit them. That's not to say that you can't make the character work, but if you want to try it you should know where the problems lie.
Drow With Two Scimitars and a Pet Panther
Drizzt Do'Urdan is so ubiquitous that he accidently became a trope. I can't tell you how many new players came into a game with no knowledge of Drizzt but still wanted to play a Drizzt clone.
If you're not familiar with Drizzt, here's the basics: Drizzt is a male drow who rejected his people's evil culture to become a good-natured adventurer. He fights with two scimitars (one which deals fire damage, and one that deals cold damage), and he has a pet panther name Guenhwyvar (which translates from several languages into english as something like "shadow" or "ghost"). There's nothing strictly wrong with any part of that, the concept has just been done to death.
If your character's personality can be described as "Wild card, bitches!", you don't belong in a party. Introducting pointless chaos makes it borderline imposible to accomplish anything, and it adds nothing interesting to the game.
The Lone Wolf
Dungeons and Dragons is a game about a party of adventurers cooperating to accomplish common goals, so the concept of a "loner" in the party doesn't make a lot of sense. It's fine for your character to not be talkative, but if they don't want to be part of the party, the party has very little reason to keep them around.