Ya'll the rock did no sins:
It woke up one day, noticed all h*l had broke loss, its fav child was at death's door n another kid died with him b4 it could do anything.
It waits till their next lives where the thread is still broken n kihoon is still absorbing murky energy. Then, by using the game it slowly helps em tie the broken thread back.
It then gives the decision of what to do back to kihoon n yeon -- leaving whether they forget past sins and events entirely up to them.
Knowing we as the viewer // yeon will likely have a heavy heart about kihoon forgetting, it decides to do a little prank n give kihoon back his memories little by little.
In conclusion: this very nice rock is now my fav char.
(Also remember that if no rock, kihoon would b single n yeon would b with the chem major(?) i forgot)
why is my bl pool drying up ( ´⊇｀)