I believe you can toggle through your weapons using left and right on the d-pad (the plus shaped pad). You can check those items there.
Another way to check is to go to the bazaar and go into the badge market. Under assist badges you find badges that upgrade the duration of the item power-ups...
Once you unlock items such as the boomerang, bombs, etc. Any character can use them. You just need to toggle through them. If you have Darunia unlocked already, you should already have unlocked the boomerang. I believe the boomerang gets unlocked during the mission before the story split in...