it's not as simple as the ML being tsundere .-. he's not even one and there's a genuine reason for his behaviour that goes beyond dumb "misunderstanding" explaining his apparent jerk behaviour. he has a demon contract forced upon him when young by a woman who wants revenge. With regards to this, the ML is blameless and the woman's hate is misdirected because no one is to be blamed just because ML's father didn't choose her. He literally can't admit his feelings for the FL because the conditions of the contract is that in return for strength and power, when he has someone he loves from the bottom of his heart, he has to kill his beloved with his own hands as payment for receiving strength from the demon. ML didn't even think anything of this when he heard about the contract as a young boy, thinking that he wouldn't love anyone. But when he does develop feelings for FL, the demon started speaking to him and telling him to kill FL as payment, and his body started moving against his own will. He started denying his feelings to the demon, suppressing himself through self-harm and avoidance of FL etc. Still, he couldn't give up on his wish to stay with FL and searched for ways to be with her despite the contract. Then the birthday incident happened and he deeply regretted hurting FL with words he didn't actually mean. He gave up on his "selfish" hope of staying by FL side, pledging never to let the demon kill her. ALSO. The contract can't be mentioned to anyone else so he can't consult with anyone else. Of course, talking directly to FL is also a no-go since his desire to kill her amplifies in her presence (there's a separate reason as to why he can later on join FL as her guard). The FL later learns of the contract by seeing his memory (an act that has its risks too).