Ace thought Luffy was annoying. He thought he was weak, annoying, and wouldn't understand pain or loneliness. He was wrong on that last point for sure.
"Being alone is worse than being hurt!" Luffy said, and his demonic features came out without his permission.
"Holy shit," Sabo said, and glanced at Ace.
Ace acted like he didn't even notice. Sabo found out that he really hadn't when Ace's head suddenly snapped to Luffy during bathtime. "Wait, do it again!"
"Do what again?" Luffy asked drowsily, his tail out and waving.
"This," Ace said and turned into a demon.
"Oh," Luffy said and his own human features changed, but not to the same extent as Ace's, "Can Sabo do it, too?"
"No," Sabo said, "I thought only Ace could."
"Gramps can, too," Luffy said, and Ace squinted at him. Apparently, he didn't know that.
"Shitty geezer didn't tell me that," Ace said, "Only told me to learn how to control it."
"Oh, that's why you can't hide your eyes," Luffy said brightly. Sabo didn't realize his eyes weren't hidden. They didn't look any different to human eyes except when he was angry.
"I can hide my eyes just fine!" Ace said.
"Nuh-uh!" Luffy said, and his own eyes faded, "Yours look like this!"
"And what should they look like?" Ace demanded. Luffy forced his eyes to look 'normal'. Sabo could only see a slight difference, but apparently, Ace saw the difference clearly.
Luffy went back to his human form except his tail.
"Why do you leave your tail showing?" Sabo asked, changing the subject.
"It's comfier," Luffy said, "But gramps and Makino said I shouldn't show it to anybody."
"Balance," Ace clarified, "When we hide our tails, we have to lean backwards to stay up."
"Oh, that makes sense," Sabo said.
When the three exchanced brotherly vows, Sabo didn't realize it, but his eyes picked up the same low-level light that Ace's had before Luffy taught him to hide it. He didn't know, but he started smiling with too many teeth when he was enraged. The people of the grey terminal called him Demon and the people of high town would come to call him fae. A changeling child.
Sabo, years later, would be told by Koala that his eyes glowed and he had too many teeth. Sabo would deny this, but smile to be compared to people he couldn't remember.
"Sabo?" The stranger from high town said, and Luffy even without knowing who he was would bare his grin of too many teeth.
"Who was that?" Ace asked.
"He must be mistaking me for somebody else," Sabo denied knowing him, his eyes glowing electric blue with fear, pain, and rage. Luffy changed to his demonic form.
Luffy grinned with fangs bared and glowing eyes and Sabo was a bit afraid of him, but even more afraid of Ace's fury directed at him for secrets untold. Later, he would realize Ace wasn't furious at him, but worried without an outlet, so he blamed Sabo's secrets.
"That was my birth father," Sabo admitted, "I was born a noble."
Luffy and Ace expressed equal disinterest in this revelation. "Oh, was that all?"
Uhhhhhhhh