Now hear me out, as many know, killjoy has a voiceline that says "My machines only malfunctioned once, and honestly, she won't miss that finger", which means someone lost a finger to her. And Reyna has a vocieline that says, "I remember the machines killjoy made, WHAT DID THEY DO". And we know Reyna has a little sister, because of another voiceline, and i believe, the one who lost the finger, could be Reyna's sister due to killjoys machine. Which in conclusion means, Reyna hates killjoy because of that.

This is just an idea, this isn't confirmed or anything, if you have any other ideas of what could've happens between the two of them, let them all out.