To note: These are first impressions and based on guesswork since we don’t know the exact details.
Specter the Unleashed is a limited alter operator who will be the third specialist [Substitute / Dollkeeper] and the first six stars. Unlike other Dollkeepers, her replacement on death does not attack directly, but instead emits an aura that slows enemies and deals Arts damage over time. Crowd control is a nice addition that seems to have tactical benefits.
One of the most obvious concerns is that as a Quirk, the new unit and the original Specter cannot be used on the same team. The original Specter is known to be a mighty 5-star unit that’s everlasting S2 can have blocking applications in even some of the toughest content. Specter the Unleashed has similar elements with her S2, in that she cannot die while S2 is active, then switches to her substitute when the skill ends (rather than being stunned). It’s still a bit unclear if this skill can truly replace the original Spectre, as skill duration, cycle time, and initial SP (for heli-dropping) are all important.
S1 is an HP swap skill that, while probably fun to use with Skadi or Surtr S3, probably won’t see much mainstream use. S3 is a burst skill that deals bonus damage to enemies whose HP is higher than itself and self-damages if it attacks an enemy with HP lower than itself. The obvious consequence of this skill is that as the enemies it attacks die, it also dies, except that its death is temporary. As a burst dps skill, its effectiveness will depend on numbers and modifiers, and will be heavily competed by the best burst damage skills out there.