Fun Fact: Sera and Arson were never meant to turn good and help Ellie. 

When I first introduced Sera and Arson, their characters were created with the purpose of being (in Arson's case) a thug (and Sera) a right-hand woman. The inspiration behind them were the two bear-brothers in Percy Jackson: Sea of Monsters. Remember them? The idiot one and the smart one? Yeah, those were Sera and Arson originally. 

You wanna know what changed that?

One word. 

Three. 

Uma's spell--- "Back to where I need to be, with my choices three". I needed a spell to get Uma off of Auradon, with Ellie. But I'm horrible with rhymes, so when I came up with that one, I decided to keep it. But then I remembered that there would be four, so I chose to leave Arson behind, thus creating Sera's sister-character-arc. 

To be honest, that might be one of my best flukes yet. Sera is amazing.