Stubborn
9 was fickle. Very fickle.
But he was also stubborn to the core.
Once he had all-too-quickly set his mind on an idea or task, nothing would ever be able to stop him, much to his comrade's dismay. The consequences of the newest family member's actions would have been considered legendary if that term wasn't a mouthful for a story known to less than a dozen beings.
That was why, despite being perfectly aware that 9 had fallen in love with her far too quickly to be sane, she still found herself believing him when he said he'd never let her down.