Exposition
K, In this version, when Roxas escaped, he killed all the nobodies, except for Larxene, Xemnas and Naminé.
He abandons the organization and takes Naminé with him.
Larxene and Xemnas, who are together, are chasing after the two nobodies to get revenge, and they're doing so with the help of a revived Riku Replica, or Repliku.
Repliku hates Larxene, but views Xemnas as his master.
Naminé realizes she has a new power: she can summon anyone from another world and gain knowledge on how to use their weapons.
I think that's the best way to explain the whole thing