1. Odysseus chooses to sail toward Scylla rather than Charybdis because Charybdis would sink the whole ship and kill everyone
2. They are like mermaids, but they're scary and want to kill you. If they sing and you hear it, you will be put under a spell and lured to your death.
3. Scylla is a really nasty multi-headed and multi-handed sea monster. The danger is that it could eat all of them.
4. Charybdis is basically a whirlpool. The danger is that the ship could pulled inside.
The author is trying to say, the person being described has two defining characters. At a point in time, he can be very lively and animated. Some other times, he is withdrawn, cool and lonely.
The figure of speech to describe this is an OXYMORON.
