The Costume Problem
Most non-human characters are humans with an appearance note. They have wings, or gills, or three hundred years behind them, and they think exactly like a modern person who happens to have wings.
That is not always a flaw — plenty of good stories use fantasy species as a visual shorthand. But if you want a character who genuinely reads as other, appearance is the least useful lever you have.
Start with What Their Body Makes Easy
Biology shapes assumption. A species that can fly does not think of a wall the way you do. A species that heals in hours has a different relationship with risk, and therefore with courage, and therefore with what they consider brave in someone else.
Pick one physical fact and follow it three steps into personality. That chain is where the character stops being a costume.
Long Life Is a Relationship with Loss, Not a Number
The most-used and least-explored trait is longevity. Three hundred years is written as wisdom and a tired voice. In practice it would mean having buried everyone you started with, more than once, and either learning not to attach or attaching anyway and carrying it.
Both are interesting. Neither is "speaks formally and sighs a lot."
Give Them One Thing They Find Strange About Us
Nothing establishes otherness faster than a small human habit your character finds baffling. Not a grand cultural difference — a small one. Why we apologise for things that are not our fault. Why we eat together. Why we lie about being fine.
One good observation does more work than a page of lore.
Let the Language Slip
A non-human raised outside human culture would get idioms slightly wrong, or use them too literally, or avoid them. Used sparingly this is excellent. Used constantly it becomes a tic and your character turns into a joke.
Once or twice a scene. No more.
Decide What They Cannot Feel
Species are usually built by addition — extra senses, extra strength, extra years. Subtraction is more interesting. A character who cannot feel fear, or cannot recognise faces, or has no concept of privacy will generate more scenes than one with an extra ability.
Write the Culture, Not the Encyclopedia
Readers meet a species through one member of it. What that person assumes is normal tells us more about their world than any amount of history. Show the assumption; let the encyclopedia stay in your notes where it belongs.
