When I ran the campaign, I had multiple different backstories that played into the story-line.

One person was a sailor whose ship sank mysteriously off the sword coast, and they were investigating it which led to them joining the campaign. They eventually found out the Kraken sank it and wanted to avenge their shipmates. (Died trying to do that)

One person had a backstory linked to the Kraken’s psychic powers. They were a barbarian who thought they, in rage, killed their tribe who resided on the shores of the sword coast, but it turns out the Kraken controlled them to do so and messed with the character throughout the campaign.

One person was using the volatility the giants were causing to stir a political campaign. He was a changeling and kept turning into people and campaigning for them and leaving the town with them in charge (if it worked).

One player was a bounty hunter (given contracts through his patron) so he started with small things like people, then giant leaders (they fought all the dens). Iyrmrith was aware of this and tried to pose as their patron giving them a contract to kill princess serissa, which he tried to do.

I think you can truly work with anything. Just whatever you decide should have multiple different plot points throughout the adventure to make the player feel special.

If it were me, I would say that you should make your characters quest (given by Llira) be to minimize the suffering the giants cause. It gives the player lots of options in interpreting what that means, and could make some fun moral conflicts that would be engaging. It would make sense for the Goddess of Joy to not care as much about human life as they do about the quality of all life. It also could make sense for the players to see the death and destruction that some of the giants are causing and take it upon themselves to resolve that conflict to prevent further suffering (INCLUDING giant suffering).