You can get Cassian’s secret ending by selecting these choices below (spoilers):
Choice 1:
"Plus, the male lead in this one is a little beyond saving..." +1
"Not a fan of getting stabbed, either."
✦
Choice 2:
"He's still dangerous."+1
"He's kind of pitiful."
✦
Choice 2:
"I can't risk changing the original plot."
"Is there anything I can do?" +1
✦
Choice 3:
"Take away the pain."
"Warm the room slightly." +1
✦
Choice 4:
"Stay completely still."
"Teach him a lesson." +1
✦
Choice 5:
"Make the atmosphere colder, sharper."
"Let frost bloom." +1
Let me know if this works!