The latest challenge event in Final Fantasy Record Keeper lets you unlock Lightning and Sazh from Final Fantasy XIII.
The event started on August 10th and runs until August 20th. To recruit the two new characters you’ll need to beat the event’s boss in the Elite dungeons. The boss has a 50 percent resistance to break effects and it’s advised that you use abilities like Boost to raise your Attack, as well as Protectga and Shellga to decrease incoming damage.
Upon beating the boss and getting Lightning and Sazh, you’ll also get their Memory Crystals, and should you use them then you can break the pair’s level caps. Find more info on the event here.