He can’t pinpoint the exact moment he realises it...
It wasn’t the first moment she challenges him before they’ve set foot on the ground, earnest and cautioning.
Nor was it the following clashing of motivations, her determination and stubbornness outweighed only by his own wilfulness.
It wasn’t when he found himself diving for her arm on instinct alone, time suspended as he momentarily debated letting her fall, calculating the pros and cons. In that brief moment, she sees it in his eyes, yet she never calls him out on it.
It might've been when she finds him in the woods, unable to do what needs to be done - he’s a goddamn hypocrite and they both know it. She does it instead, with only a tiny hitch in her haunting melody, and he’s in awe. She doesn’t hold it against him and he begins to believe.
(They aren’t friends but they’re something)
Maybe it was the unplanned weapons lesson in the bunker when he corrects her stance, her posture, and his hand lingers on her of its own volition. When he snatches it away, he’s thankful for the darkness underground covering his unease, and can’t help smiling at her almost child-like delight when she succeeds, so at odds with her inherently diplomatic nature.
It could almost definitely have been when she saves his life, and in the ways that count, his soul, offering him forgiveness, unconditional and unwavering, He knows he doesn’t deserve it but he takes it, not for himself - for her - because no one, aside from his mother and sister, has ever said they needed him in the way she does, raw and genuine.
No, Bellamy can’t pinpoint the exact moment he realises...Clarke is the sparks and spaces between his cells...