An almost indestructible one-hit-point spirit normally dismissed as harmless because it cannot meaningfully act.
Mordecai describes withering spirits as indestructible and insane but generally non-dangerous, lacking the ability to affect their surroundings. Samantha is violating that expectation: she can bite, roll ever faster, activate missiles, and retrieve people through Carl's xistera.
Her growth makes Princess Donut question whether the usual species rules still apply.
The form can inhabit a severed artificial head with only one hit point yet survive fire and explosions.
The creature classification establishes the baseline Samantha is rapidly exceeding.
Also appears in books you've already read — spoiler-safe to their endings: