Why Stella & Loretta? They are gimmick characters that float around.
How dare you say such things! ><;
Stella and Loretta are important characters in Portrait of Ruin, and they are practically the first villains in the series history to have their own story-driven playthrough, which is a playable prologue that is canon to the main story!!!!
They even included character interaction scene before and at the last part of their gameplay walkthrough, so it isn't just a tacked on mode like Richter mode in SOTN. Sisters mode perfectly adds more to the story and its a rare to offer a complete playable prologue through most of the locations like this.
I admit the stylus gimmick was a bad design choice for Sisters mode (the STYLUS is the gimmick, not the characters),
but the mode still technically counts Stella and Loretta as part of Castlevania's long list of playable characters in the series with meaningful and unique story background on top of that, and characters that played the role of BOTH protagonists (in their own story) besides antagonists in what's possibly the most tragic story in the series history!!!!
They really deserve more credit from Castlevania fans, it's great they are included in the top of this page along with the other Castlevania heroes. They are more than NPCs, and more than bosses. They are also Castlevania heroes that tried their best to rescue someone important to them.
There are many good ways they can adapt their gameplay into HD. For one thing, flying can be rebalanced so it can't be used as often or as long as it could be used in Sisters mode. It could drain a lot more MP than before and could be used as a Personal Skill in short bursts.
Replacing the insane attacks they had using the stylus, Stella could be given her boss-based slash/wind specials as Hunter Skills, and Loretta could be given multiple ice spells as her own Hunter Skills. (yes there are already characters that use ice, but they can design it so Loretta is a master of ice, so she could be given a unique spell or two.)
And they could have their own unique Dual Crush together. They already have sprites that can be used as ducking and dive kick type actions. And adding a slide animation wouldn't require a lot of editing.