Adamantine – utterly unyielding or firm in attitude or opinion
Sirius

)O(

"Stubborn" was often the word associated with young Sirius Black.

He accepted this – embraced it – and lived up to every expectation. He defied his parents, he decorated his room with Muggle pictures, he proudly wore his Gryffindor scarf and tie to family dinners. But he never did anything really terrible.

At least, he didn't until he got the idea into his head to run away.

Secret-keeping was not a strong point, and it did not take long before his cousins – the two that were left, at least – and brother were well aware of what he was planning to do. It surprised him for years afterwards that they never told his parents, but perhaps they simply did not expect him to follow through.

But he did.

Sirius was adamantine in his decision to leave, and no amount of begging and pleading from Regulus or Narcissa, or threats from Bellatrix, could make him change his mind.