I don't think Kairi can weild the keyblade...

[Chuman]

Kairi was able to wield it in KH2 so i dont see why she wouldn't be able to wield it in KH3 and future titles. I mean there is a good reason why Yen Sid wanted Riku to get Kairi to him (which brings up the question how the hell Riku was able to travel from Destiny Islands to Mysterious Tower since we saw he cant use/open a corridor of darkness anymore after they defeated Armored Xemnas 1 and i dont think he has access to a gumi ship).
But sadly Kairi doesnt have enough relevance to the overall series up to this point, except that she is one of the seven princesses of pure light. Maybe she will gain more in BBS 0.2 and KH3.

he could have

a. used a star shard
b. used sora's ship while he's in the RoS
c. used a keyblade glider
d. opened a corrider because he's embraced the darkness (unlikely but i wouldn't put it past nomura)
e. yen sid could have willed them there in puffy blue smoke