What makes a character a character, how he says things or the sum total of his experiences?

As a machine presumably his voice, his general demeanor etc could be adjustable (iPhone's Siri can has different regional voice styles and that's a dumb mobile phone personal assistant).

As a character (one we are clearly meant to care about like Yoda or Han) Threepio is the sum total of his memories.

That's what his life is.

Wipe that and what he was is dead and he becomes something else.

If you just twiddled his voice setting he would still be the same droid with the same memories.

If you keep the voice but remove the memories he becomes a blank slate with the same voice.

If Leia had all her memories removed you'd care right?

So why not Threepio?

One is a droid, the other is a human but the OT as framed told us that we should care about him as much as her even if she doesn't.