...habitually finishes your sentences for you? I dunno, but Onelook's Reverse Dict. certainly provides some tempting suggestions!
http://www.onelook.com/?w=*&loc=revfp2&clue=someone+who+habitually+finishes+your+sentences+for+you
If that link makes anybody's screen go wide, holler* and I'll change or delete. I noticed a couple of earlier comments about it going wide, but in neither case was the screen wide for me.
*Send me a PM or an e-mail; that way I'll see it quicker than I might otherwise would.