(Info. from Fire Emblem Wiki)

Limstella is the most powerful of the morphs created by Nergal, and ostensibly the most powerful magic-wielding foe in the game other than Nergal himself. It shows few true emotions and is blindly loyal to its master. 

Though one might guess that it is compliably without emotion, a few instances where it gives some reason to believe otherwise: in appearing to take a sadistic pleasure in revealing to Sonia, who believes herself human, to be a morph herself; and upon dying, it says: "I am not human; this mind and body are constructs, yes, as is this sorrow."

Overall Limstella seems indifferent to almost everything, even its own death. Doing little to harm the protagonists, or even neutral units (except for a few fallen Black Fang members) until near the end of the game, it seems to lack the same sadistic nature found in the other morphs, like Sonia or Ephidel, and only seeks to harm those who it directly ordered to attack. Strangely, despite being the most powerful morph, Limstella also seems to be the most docile.

Limstella's first priority is to collect the quintessence of the strongest individuals for use by Nergal, and thusly it is shown to harvest the essences of many of the latter boss characters, apparently also having taken the quintessence of Marquess Darin offscreen. Although it is feminine in appearance, Limstella is an asexual entity whose body eventually would have been corrupted by the power of the magic it wielded were it not first defeated in battle, after which it promptly disintegrates into nothing. Completely devoted to its master, Limstella will sacrifice everything for Nergal, even its life.

Nergal claims that Limstella has acquired "perfect power and perfect beauty" before sending it forth to dispatch the protagonists and claim their essences. It is Limstella who reveals its acute awareness of its kind to the protagonists with its dying words, "This body and heart are constructs. Yes, as is this sorrow."

Now, for her stats. Limstella's stats are INSANE, practically. Ever heard of a Sage with insanely high Defense, Resistance, HP, Magic, Skill and Speed all rolled into one ominous cloud? Thankfully, her Luck is a miserable 0, so your units can deal critical blows almost all the time.