I was the first by his side, but I always worried he'd lose track of me. I'm smaller than the others, quieter, and I don't like to fight for attention, eh?

[It's clear from his tone that he'd really rather Zoro never repeats this to anyone ever, but Marco hopes it helps, so for Zoro he'll share it.]

But he told me he could always tell when I'm around because everyone is more at peace, eh? That's why he trusted me to look after the others no matter what. I thought I had to compete with them, but instead he was trusting me with protecting his treasure, see?

Sometimes when you can't see the effect you have on others, it's because it's in the exact opposite direction from where you're looking.