Learn Hardcore Turkish: O, kaba biri değil; tam tersine çok kibar ve duyarlı. - He is not a rude person; on the contrary, he is very polite and considerate.

bold O is the third-person singular pronoun and is gender-neutral: it can mean bold he or bold she. It can also mean bold it for things, but in this sentence the context clearly points to a person.

Is the comma after bold O necessary or normal?

It’s optional and stylistic. You can write bold O kaba biri değil … without the comma. Many writers prefer a comma after transitional expressions like bold tam tersine instead: bold O kaba biri değil; tam tersine, çok kibar ve duyarlı.

Why is there a semicolon? Could I use a period or a comma?

The semicolon neatly links two closely related, contrasting clauses. You could also use:

A period: bold O kaba biri değil. Tam tersine, çok kibar ve duyarlı.
A comma (common in Turkish): bold O kaba biri değil, tam tersine çok kibar ve duyarlı.
A connector: bold O kaba biri değil ama/ancak tam tersine çok kibar ve duyarlı.

What does bold kaba biri literally mean, and why not bold kaba bir?

bold Kaba means bold rude. bold Biri is a stand-in noun meaning bold someone/one (person). So bold kaba biri ≈ bold a rude person. You cannot say bold kaba bir by itself because bold bir is just the article bold a/one and needs a noun (e.g., bold kaba bir insan). Using bold biri conveniently fills that noun slot.

Can I say bold kaba insan instead of bold kaba biri? Any difference?