They manifest in the form of a small, partially decayed, maggot-riddled child with gleaming, intelligent red eyes. Adorned around their neck is a memento of their time with Frisk. Said memento is a tattered, dusty, spectral scarf that once belonged to a certain skeleton.
When Aliza fell into the Underground, Chara saw great potential in her. But the girl would not be swayed by their honeyed and calculated words. For all the power Chara has gained in the years following the Hunger, they remain unable to wander far from the flowerbed where their body was buried so many years ago without a host.
So when Aliza slipped beyond their grasp, Chara tore off their own smile, giving life to a heinous, moth-like creature, their (literal) mouthpiece capable of following Aliza on her journey to attempt to coax her into actions that would give them a foothold in her soul. Should that happen, it would spell doom for all.
Their power grows the more death is dealt, and they have become almost as corporeal as Napstablook in appearance. However, they have no power without a host, and require a body to wield any of their malicious designs. The monsters remaining in the Ruins are too weak for their uses, those stronger staying far from the haunted grounds of the Flower room, and their influence is limited beyond a few hundred feet from their rotting corpse. And so they seethe, and linger, and plan, and wait for another weak-willed, easily twisted human to fall into their grasp.
They are very seductive, charismatic, silver-tongued, and persuasive.