Star
Ace was a star.
A bright, blinding light.
Always ahead of Luffy.
Showing Luffy the direction he had to go.
It didn't matter that Ace left Dawn Island earlier because Luffy could always see the light of his brother on the horizon guiding him – like a star.
Of course, Luffy was going his own way. So, he sometimes strayed from his path, but Ace's light was always there to guide him just like he did as they were kids running through the jungle.
And suddenly, the light disappeared.
The star vanished before him.
Ace died.
And Luffy was lost.