The life of a pirate was always unpredictable but one thing to be expected was there would be surprise attacks on the ship, even if those attacks were rare ones. There were still some people that thought that trying to take on the moby dick was a good idea though Ace himself didn’t have much room to judge there considering how he was when he first joined the crew. However, there was much more to worry about with a small child like Aiko on board when there was a surprise attack. That’s why Aiko had been put into his arms while everyone else was fighting off the crew, the fire user taking a backseat approach to this as he only used his devil fruit when other pirates got too close to him and Aiko. Not using his full power of course but just enough to blow them back and into the water, Keeping Aiko firmly in his grip to silently reassure her that it would be alright.
What happened afterwards is what got Ace more mad, watching Aiko biting both Thatch and Marco when they tried to take her back from him and he had to carefully pry her little teeth off of them, hearing the small growls coming from her. Normally he would think that she was just startled from this being the first fight that she had been exposed to but considering that she bit both Marco and Thatch there was only one other explanation as to why she had this reaction.
“Neither of you slowly exposed Aiko to your devil fruits did you?”