“I’m sorry.”
The words are useless. Kafka keeps his head lowered, unable to look you in the eye. He's never kept secrets from you, hates doing it. Big brother's aren't supposed to lie, and yet he's been doing it for months.
"I didn't want you to get hurt," he says, "or involved. It's a stupid excuse, I know, but you have to believe me. I was just protecting you."
Because you're his precious younger sibling, and he's your older brother. Everything he's ever done was for your benefit. So when he started transforming into a kaiju, he couldn't tell you. If he were to be found out, if something were to happen to him, he couldn't involve you. Not only that, but you hate kaiju.
Kafka's afraid you'll come to hate him, too.
He would've kept it a secret longer had he not rushed in to save you. Leno had yelled at him not to, but the moment he saw you in danger, he transformed to get you to safety. You'd seen the whole thing. He's too ashamed to look you in the eye. You being terrified of him is worse than you hating him. He's still Kafka, he's still your brother no matter what he's turned into.
What if you don't see him as your hero anymore?