Honestly I kinda get why Sejin won't stand up for himself, he's so emotionally repressed.
He's been controlled his whole life and treated like a tool. It's so ingrained in Seji that his purpose is to follow the orders of others, not cause a ruckus and be seen not heard.
With the memories of his 1st life making him unconsciously remember the trauma in that life.
It does frustrate me, but u get it! Still ugh also the ML is on the better side of other ML's in this genre and at least he is making an effort for redemption not like certain ML's I know ( ̄へ ̄)