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One of Sanji's secrets which he has many of, is that he always hears Luffy's heartbeat.
It's always been bizarre to him, since the day, Luffy's been in baratie; it's been this way.
Though Sanji doesn't know if it classifies as a heartbeat. Observation haki is all feelings and experiencing them, and Luffy's heart to Sanji sounds like the beat of a drum.
Sanji only taps into focusing it only when he needs to find Luffy, especially when he's drowning, it's just easier to focus on the sound rather than concentrate on his form. It's ever present, it thrums under his skin. Initially it made Sanji feel alarmed but eventually he had gotten used to it.
After the two years at peachy island where he learned observation haki, it was quite intense. Luffy got stronger, so did his heart and Sanji got stronger and so did his senses.
Changes still happen in that beat, when Luffy's soul deflates at whole cake island as Sanji leaves, he swears he will starve himself and he does.
In the rain Sanji searches and searches screams for his name, the familiar thrumming is lower than his own heartbeat and Sanji refuses to give up and he is rewarded for it with Luffy's presence.
But now as he laid unconscious in a burning building being nursed back to health by Osome, the intensity and volume of Luffy's heart becomes deafening, wakes Sanji up in panic. It felt as if the earth was shattering right under him and the thing he focuses on is Luffy.
Luffy who laughs and laughs so hard that his throat hurts, entering the awakened state of his devil fruit; taps into whatever is required to be able to bend reality to his will.
"Luffy" Sanji says breathless, looking up at the source of this bizarre intensity, the woman beside his squeals in fear but he's more worried about what state Luffy must be in for this to happen.
Worries swim in his mind, a similar thing used to happen when Luffy shifted to gear 2nd for the first time, his heartbeat growing fast as a result gave Sanji a massive headache but now it's so loud he feels like his eardrums will burst.
He starts getting up almost in a trance, the sweet lady beside him hands him his clothes.
"We need to get out of here, what the hell is happening,"
Sanji says his head feels like it's going to explode but he knows, it's just him. No one else at least not in floor he is at can hear any of this.
"He'll win, he'll be okay we just need to get out of here,"
Sanji assures himself. But it's scary, the laugh and the beat reverberates in his mind insistently.
Eventually as they do find an exit, the island floats slowly and Luffy's heartbeat is back to a somewhat normal rate, and Sanji heaves a sigh of relief.
A party eventually takes place in wano and Sanji forgets all about it. But something is off, somehow Luffy's presence is a bit different, familiar yet he keeps changing and so does Sanji. He feels so accutely aware of it, his mind gnaws at him for an explanation.
At egghead island Sanji finally gets to see what exactly puts Luffy in that state, a pure white form. White clothes, white hair and swirling smoke clouds.
Vegapunk says it's the form of a God. And Sanji rejects the though of it.
" A god in the oldest texts, a devil fruit lost to time,"
And Sanji believes in the freest god of the seas, the future pirate king. The future so close he can almost hear it thrumming under his skin.