Luffy wears his hat because it was from Shanks. Chopper wears his hat because it was from Hiruluk. So maybe Robin wears (or at least used to wear) those cowboy hats because they reminded her of Saul.
Along with safety, food, shelter, and sleep, clothes was another issue while on the run. Robin's quick growth had proved to be a huge problem to the matter, and she usually ended up resorting to stealing from clotheslines or stores. Not that it affected her conscious, or anything. After all, the people she stole from were just among the Government-run bastards who were constantly trying to capture her.
Most of her time had been filled up with running or hiding. It was a never ending game of cat-and-mouse, except there was only one mouse and a thousand cats. Run, hide. Run, run, hide. Hide, run, run. Hide. The pattern varied, depending on where she ended up.
Robin had only been ten at the time. The shirt she wore was much too large, and her shorts were crudely sewn from the remains of an old rucksack. Desperate times called for desperate measures. Except this time, for a brief five minutes, Robin took one of the largest risks she had taken in those two years. All for a stupid hat.
That hat had cost her a knife wound to the leg, but it was still worth it.
It was Saul's hat, that was why.
No, it wasn't his. But it was the same design, color, material. It was the cowboy hat that Saul had worn as he laughed and smiled.
Robin held onto that hat, half running, half limping away into the forest as the heavy footsteps faded farther and farther away. It was much too large for her, but she wore it anyways. She found a tree stump and curled up on it.
"Dereshi." Robin forced a grin on her grimy, dirt-encrusted face that had been etched into a seemingly-permanent scowl of fear, sorrow, anger, and distrust over the past two years. She pulled the hat down lower over her eyes, and imagined she was back on Ohara. That Saul was still alive. That the Tree of Knowledge still stood above the island, tall and powerful. Like Saul.
If you aren't happy, just laugh...
"Dereshi!" Robin tried again, stretching her smile to the point where it hurt. No, it didn't work. She wasn't happy. She was alone, scared, tired, hungry, filthy, wounded, and downright miserable. But for those brief seconds, with the worn out cowboy hat over her eyes, a flicker of peace had overcome her stressed mind.
The hat didn't last much longer, especially since a few bullets had gone through it. Robin was alone again. The fear and loneliness must have snapped something within her, because when she left the hat, it felt as if she had lost Saul again.
Maybe she was going crazy.
Hearing voices in her head.
Seeing laughing giant faces.
Imagining that those who were dead were still with her.
People didn't come back from the dead.
So she was going crazy.
Robin found another cowboy hat in the next town she visited. She stole it from a clothing store, along with a few shirts and jeans and a pair of boots. The bullets had narrowly missed her head, but the nearby mountains provided shelter. Robin quickly tore off the patched up rags she had been wearing and pulled on the stolen clothes. The soles of the boots rubbed and chaffed against her blistered feet, but it still felt nice not to be barefoot for a change.
When she put the hat on, she saw Saul.
Run, Robin, run...
People didn't come back from the dead.
The insanity was slowly taking over her. It was inevitable.
In her sleep-depraved mind, Robin imagined that the hat provided comfort. It was a remembrance of a brief few days of happiness she had.
Saul was dead. Olvia was dead. Everyone she knew and loved was dead. Robin had to move on and survive.
But it was too painful to forget.
The hat made her remember.
"I don't remember getting you that." Crocodile rose an eyebrow, smoke billowing out his nose and mouth.
"I bought it myself." Miss All-Sunday muttered, hands jammed in her white fur-lined coat's pockets.
"Hm. I guess it goes with the outfit." Crocodile swept his eyes up and down his partner's image. "Fine. Keep the hat."
Sixteen years, and she still wasn't able to get over what had happened so long ago. Was it a sign she was weak, sensitive, vulnerable? No, Miss All-Sunday didn't like to think so. She was simply... honoring a friend's memory.
She still didn't trust them. She never would. If she let her guard down, they would take advantage of it and stab her in the back. Just like everyone else did.
Was it normal to feel so lonely for so many years?
Or maybe she was still a little bit crazy.
"I didn't know you still had this." Nami said incredulously, pulling out a white fur-line coat deep within the chest. She fingered the hole in the right side of the coat. If she didn't know any better, she might have imagined it smelled a bit like blood and sand.
"I didn't know, either." Robin said in mild surprise.
"Hm." Nami nodded vaguely and continued digging through the chest. "Oh, wow. There's a lot of old stuff in here. Here's that shirt I wore when I met Luffy- it's way too small for me now... and that skirt I wore in Syrup Village..."
Nami pulled out a dusty cowboy hat. "Hey, Robin, I haven't seen you wearing this lately. How come?"
Robin stared at the hat. She remembered how it had provoked a memory to appear in her mind, how it had brought a little peace and nostalgia. It was a remembrance, a memory, an anchor to something she had refused to let go of for so many years.
But now, she had found her place. Saul hadn't been lying after all.
"I don't need it anymore."
Nami shrugged and put the hat back in the chest.