The manufactured radianite the machienes Killjoy invented for the task responded badly to the living radianite in Reyna's sister's heart and caused an explosion, which killed Reyna's sister.

Why does Reyna dislike Killjoy?

It is hinted that, at some point, one of Killjoy's inventions was responsible for causing harm to someone that Reyna held close (most theories usually label this 'someone' as Lucia). Killjoy, for her part, is mostly oblivious to the fact that her creations could be used for such a purpose.

What is Killjoy's real name Valorant?

Killjoy – Klara Böhringer (found on Haven)