Scylla, the "savage, extreme, rude, cruel and invincible" monster who eats crews six at a time is still the better option than her sister monster, Charybdis.
Scylla and Charybdis are the worst of the feminine dangers Odysseus faces on his trip home: they are the final trial after leaving Circe's palace and passing the Sirens' isle. If Circe's island and the Sirens represent temptations of the flesh leading to death if not resisted and controlled (gluttony, drunkenness, and sex), Scylla and Charybdis are the inescapable power of Nature, often represented in Greek myth as an impersonal feminine rage. Neither Scylla nor Charybdis were out to personally murder Odysseus: both simply are what they are. It's up to Odysseus to take care around them, for neither care in the least about him and will continue to act accordingly to their natures regardless of his presence. It's a good lesson for Odysseus in humility: there is NO way to conquer either creature. He can only hope to survive. And survive he does: against all odds, he does it twice (although the second time Odysseus's raft is swallowed by the whirlpool and he finds himself clinging to the fig tree above her, watching as his little vessel is destroyed and belched up in bits).
The Odyssey doesn't specify any physical attributes of Charybdis other than her tidal powers of dragging ships to the bottom of the sea thrice daily. I suppose for a sailing culture her destructive powers are terrifying enough without necessarily discussing the actual creature causing the whirlpool, since by design no mortal would've ever actually seen her.
It's possible Charybdis was once considered a goddess of the tides. Aristotle refers to her as such in his work Meteorologica, and a her name is linguistically similar to "Keto Trienos" (Sea Monster, Three Times). Later, in Virgil's The Aeneid. she's described as the daughter of Gaea and Poseidon. That'd be the Great Mother Goddess (Earth) and the God of the Sea. Ah, incest in Greek Myth (Gaea was technically Poseidon's grandmother, after all).
As most monsters prove to be, Charybdis is one of the faces of the destructive power of the Goddess. She is described unrelenting, voracious, and unapologetic in her hunger. She stole oxen from Heracles and was banished to the bottom of the sea by Zeus's thunderbolt as punishment. In some versions of this later myth, Charybdis is a lovely girl and loyal servant of her parents who, in her punishment by Zeus, is turned into a giant "bladder of a monster" with flippers and a voracious thirst which could only be relieved by swallowing the sea three times daily.
It's worth noting that Charybdis appears in all three major hero quests in Greco-Roman literature: The Odyssey, The Aeneid, and Jason and the Argonauts. ONLY Jason and the Argonauts are able to pass the straits safely, because they carried Thetis with them as a guide. Yes: that's the same sea nymph Thetis who birthed Achilles and was the daughter of a sea god, but her tale is another post.
Charybdis still haunts the Straits of Messina as the natural whirlpool on the northern end of the strait. I'm sure the sailors today haven't seen the creature herself who resides at the bottom of the sea there, but some of the art out there depicting Charybdis is fantastic and worth looking up. The watery sarlacc pit with teeth is my personal favorite. Since the tides still whirl in that spot three times a day, I imagine she's still there, waiting for ships to sail too close to the edge.
Charybdis depicted in Percy Jackson: Sea of Monsters