I've had a similar situation. I got the boxed message "THE DIETY RIITA HAS COME. A GIANT AS TALL AS THE MOUNTAINS IT WAS NAMED FOR, WITH LEGS THE SIZE OF LARGE TREES AND ARMS RESEMBLING SLIGHTLY SMALLER ONES. ITS SUNKEN EYES, THOUGH SHOWING LITTLE INTELLIGENCE, REVEAL UNRESTRAINED HOSTILITY TOWARDS ANYTHING THAT MAY BE CONSIDERED FOOD, NO MATTER THE SIZE!"
So yeah. I didn't have much of a military (this was in maybe the second year of my fort) and I did what any sensible person would do: panic, pull all my drawbridges up, and wait. Oh, and start making crossbows. Lots and lots of crossbows.
I had those buggy traders on my map (maybe 5 of the stupid things) that appear with migrants and block the flow of migrants sitting out there, and I gleefully thought "Oh, they're gonna die!" but Riita never attacked them. Eventually I checked my units screen and saw she was friendly and cautiously lowered a drawbridge. She walked right in, and has been perched in my living quarters ever since.
I happened to check the Frost Giant Civilization and she is listed as a leader of the frost giants (there is one warlord--not her, and a titan which is her). She's been chilling out for the past 3 or so years my fort has been running. Doesn't do anything, doesn't say anything. Just stands there in the same spot.
I have no idea what will happen when the frost giants start ambushing my dwarves or if they will with their leader camped out in my fort.
It's so friggin' weird.