REGISTER TO REMOVE ADS
I'm assuming it's Marluxia since the two of them had some sort of connection back in the Chi era. We don't have much to go on with it but there's a scene in KHUX with their Somebodies. So far that's all we've seen of Larxene's past.
https://youtu.be/hp5UopFGFsk if you want to see the scene in question(along with other plotty stuff), although you can skip over all of it and just go to about 8:25 of the video if you just want to see that since the rest might be confusing if you're not caught up with that story.
Oh wow, thank you! I didn't realize that Elrena was revealed! When they mentioned that Demyx, Luxord, Marluxia and Larxene were chosen because of their history with the keyblade, I was so shocked (well I knew about Marluxia).