The first time Brook sees them will forever be engraved into his mind.
He's hiding, buried under rubble, watching with bated breath as the green-haired man, his saviour, walks with the giant bear-like man. A warlord, his mind supplies the words that the young orange haired lady uttered in fear. He doesn't know what it means. He doesn't know what it signifies. But he can feel the danger it holds. The danger becomes visceral as he sees the young flailing cook grab tightly -like a lifeline- onto the swordsman's arm; almost begging him to stay alive despite the harsh tones of his voice. And the danger glows and rackets in the young man's body.
Extracting himself from the rubble and mayhem he follows the warlord and the swordsman, Zoro, to a secluded area. The gruesome cuts and muffled screams come to an end with Zoro's stock-still stance, crossed arms, and glazed eyes -and a scene of bloodshed. The man is ready to keel over when Brook sees it.
A small and pale blue light reflecting off of the green-haired man's earrings. The light hovers and slowly inches to the front of Zoro's face. The light morphs and melds into the shape of a young woman almost as tall as Zoro himself; draped in white robes splattered with midnight blue hues. Her hair a matching colour held by three golden pins resembling the swordsman's earrings.
She has her arms coiled into fists and repeatedly bangs on the wounded man's chest. Brook is about to get up from his hiding when he spots her face. Tear-tracks mar her features as she continues to pound on Zoro's chest and her mouth open and close as if shouting at the swordsman. Zoro is completely unaware of the woman in front of him. That is when Brook notices that the world around him is silent; and cotton seems to be plugging his hearing because she is shouting and yet he can't hear.
Brook strains his practiced ears to hear something -anything- that the girl is shouting but all he hears is deafness. The pounding continues and Zoro remains unperturbed. Until...until he hears her hushed voice; seemingly exhausted from the pounding her fists weakly grab purchase onto the torn threads of the swordsman's once-shirt and her head lowers into his chest, and Brook can make out a few hushed words. "Not yet...our promise...you can't come here...not yet...not yet."
Brook is too struck by the raw distressed tones in the girl's voice to register Sanji's shouts and subsequent run towards the swordsman. But he notices when the girl shifts her body and moves to kneel in front of the screaming cook; holding onto the charred pant leg of the cook like a child -and she has digressed into the form of a young child with robes too big- she sniffles to the unhearing cook: please. Then she's a blue light again and that's when Zoro finally closes his glazed eyes and keels over to the ground.
It takes a while for Brook to realise that nobody but him can notice the embers of light and a while longer until he learns what they signify. These embers of past people only appear rarely around people but when they do it's heart-wrenching to watch.