He’s Joffrey’s dog. Mean and biddable. At the beginning he’s just a tool, and that’s how he sees himself. His will is not his own. Just because he’s a servant, though, he’s not a simpleton. He sees The Game around him and understands it. It amuses him. Killing and laughing, really, are the breadth of his range. He’s quite interesting as a character study.
Ah, he’s the first person to challenge Sansa, to point out that she’s a puppet. She has not begun to discover herself but only behaves as she’s been trained. Why does he care enough to confront her with this?
And the story of Sandor’s burning is told to Sansa by him, not by Littlefinger’s gossip, here in the book. Why diminish such a powerful story, such a powerful moment between these characters, by giving this speech away? He threatens her to secrecy later, not that she would tell anyway, afraid possibly at the vulnerability he’s shown. His behavior is a wonderful mystery that makes him very interesting.
When he jumps in at the tournament to rescue Loras from the Mountain he’s magnificent. Brave, obviously, but he’s also a kind of watchman on his brother, who’s a truly evil creature. Stop Gregor from hurting anyone else might be Sandor’s only motto.
Now he’s part of the Kingsguard, standing watch over Joffrey. At the “gnat’s” tournament he takes Sansa’s part, although with a stone face. He won’t hit Sansa, the only guard who isn’t challenged to do so. He doesn’t care about anything, it seems, yet he interjects himself so effortlessly at key moments.
The ironic detachment, the physical competency, the lack of will to create his own destiny — what number? Nine, of course. He avoids conflict by not caring about anything. Only Sansa, through innocence, vulnerability, or naivete, can get him to engage.