Been playing in offline mode, can I update key and keep my character?

sorry I have no idea about this T__T

So as long as your GFWL (aka xbox live) account is the same, you can keep your character

If not, then I wish you the best on your new journey. Should you wish to start again of course

I also went through this, I didn't have a key (or account) of my own so I played offline using my brothers copy. When I decided to create my own xbox account, I could do nothing with my old offline saves. Luckily I have console experience so the grind back to glory wasn't as tough as expected.

If there is a way to link the offline account, or turn an offline account into an online account (assuming you don't have a xbox account yet) then your saves should still be usable