The Building Blocks
I’ve put together a cheat sheet based on how I begin building my characters. There’s tons of those out there already of course and a lot that go into details 500+ questions deep. But we’re not looking for a laundry list of How many siblings do they have to How do they cross the street here. Rather, we’re trying to build a foundation from which the rest of the character can grow comfortably.
It covers things like their physical description, their voice, and (most importantly) their motivation. All things you need to get you started.

Who are they?
Begin by jotting down three words that represent them. Don’t overthink it. Boil them down to the essentials.
Physical:
Description. Notable Features. How do others see them?
This may be a misleading section. While description and notable features are largely straight forward, How do others see them? is one of my favourite questions to ask. Do they easily intimidate others? Are they often underestimated? If so, why? What characterizes them to others and how does the world at large treat them?
Voice:
Vocabulary. Speech Pattern. Thought Process. Ticks.
This is where the character’s voice comes from as you write them. Think about what words they use a lot. Or if they favour simple words or big words. Dig for ticks that may bleed into the narration. Do they think a lot? What do those thoughts look like? Do the ramble or keep things precise? Do they drive past a pasture and go Horse! no matter what? Those sorts of things.
Motivation:
Goals. Why? Overcome
What are their goals as the story begins and as it continues or concludes? Why are they doing what they are doing? And what do they need to overcome?
This, in particular, is the part that will change over time and show character development. Though, really, all of the above can (and might) change. Being consistent as you build them is important so that when the change happens it matters.