There are so many options when making a character that it can be overwhelming, and this can lead to frustration for new players. So instead of deluging players with all the options in the rulebooks, invite them instead to imagine a character and then help them learn to use the rules to flesh out those ideas.
The only limitation to character creation should really be whether the character concept fits into the setting and genre of the game world.
“What character are you going to create for our grim medieval world?”
“My character is a plucky green alien monkey who uses their bounty hunter training to help the poor.”
So maybe it’s possible that the character concept doesn’t mesh with your vision of the game setting. Still, sometimes this clash of ideas brings up interesting new possibilities for storytelling. Consider how this seemingly strange character concept might fit into your world. Sure, your world doesn’t have any alien influences. But are there other planets in your universe? Is it possible that an alien could have landed on your world? Try to work with the player to find a compromise that works for everyone. Perhaps, instead of a green monkey, the character is a humanoid bounty hunter who escaped their employer by hiding out on this previously uncharted world.
Now that you’ve got a character concept, you need to help the new player adapt that concept to the rules. You don’t want to stray too far from those rules though because they’ve been balanced and playtested to maintain a fair game for all players. So you don’t need to write an entire new race for this character and can just make modifications to an existing race when necessary. Find which race is closest to what the player envisions, perhaps human, half-elf, or half-orc. Use minor modifications, such as a swim speed, and avoid potential game-breaking abilities. You can add things that will help adapt the alien to your world. Maybe the alien is able to disguise themself as a human or half-elf, but they can’t change their body shape or likeness to a great extent. Adjust the ability score modifications, but stay within reason, keeping the sum total ability modifications the same as the base race. Of course, this modified alien race need not really be related to an orc or whatever race you use; you’ve simply used those statistics as a template.
At this point, before deciding on a class, you should look at the background for this character. Deciding on a background will let the player answer some questions about why and how the character got to this world and region, helping to determine which class best fits the character concept. We know they are a bounty hunter, but what made them good at it? Were they a survivor who could live in any environment? (Perhaps a hermit.) Were they abandoned as a child and forced to live the bounty hunting life? (Perhaps an urchin.) Get the player to talk about who the character was in their youth, and a background will likely emerge. As with race above, you won’t need to stray far from the rules. The medicine and herbalism the character learned, for instance, may have been from another world’s environment, but they will soon adapt it to this new home.
All this should help decide what class best fits this character. As a bounty hunter, the character could fit into many character types. What skills did they learn in their early background? What skills did they survive on as a bounty hunter? Did they learn to use subterfuge to survive? (Perhaps a rogue.) Were they able to track down their bounty in any environment or situation? (Perhaps a ranger.) Did they survive by being tougher than their environs? (Perhaps a barbarian.) To reiterate, you should stick to the rules as much as possible, only adjusting where necessary. Use the character’s story to explain their skills, making a complete character and backstory at the same time. If the character’s class has enhanced skills when combatting a particular type of monster, it could be that this monster type is similar to a creature they were familiar with on their home world. If the character can do additional damage when flying into a rage, then this could be due to something about this new world’s atmosphere. You can use the exiting skills by simply modifying the story about how the character acquired them.
Learning all of the rules of a new game is not always fun, and that includes the many options in character creation. But playing the game should be fun. Letting the creation of a character’s story define their character’s statistics is a great way to make the character creation process fun rather than a chore. New players will end up learning the rules as they create an exciting story.