This works for all the characters.
1st get your team to 4star rank (you can see your progress on the right hand side of the character info screen) do this by beating story and side missions. Raising your team rank allows you to then raise your characters ranks.
Always save before starting a mission and keep trying it until you complete all the additional requirements in the quest to get the maximum rewards to get your team rank up faster.
Now, for your characters 4 star rank upgrade, you'll see it says you must complete the specific quests. To do that go to the map screen, and switch characters with RB/LB. You'll notice the Side Quests displayed around the map will change with each character you switch to. Go do all of each characters side quests to unlock their respective upgrades. If you've done all of them and still can't upgrade, you may need to progress further in the main story to unlock more map and more side quests.