Callisto

I think I get why the manhwa is called "Believe My Sign", because Kang Eunchan can be quite harsh with his words but it is evident that he truly cares for Ji Soojeong through his actions. There are a lot of "signs" that shows he also likes soojeong as much as soojeong likes him. And we will definitely see that later on.

perhaps he just said his password out loud to everyone in open, it just that nobody know it's the password