SPOILERS IF U AINT READ IT YET BUT!
jinsol found a way to exchange the ability of truth or dare by kissing another person. when he found out saegye was a dare, he wanted said ability. but jinsol was not a truth like we thought. he's a false. so instead of being able to only tell the truth, he can only lie. now saegye cant tell the truth, he can lie. he's a false. the whole "i love you" thing was a way for him to test his stuff out and also see if he can become a dare (him saying i love you then saying the headache stuff was a very real pain) when he asked saegye to say i love you, saegye wasnt able to bc he does love jinsol but he can only say lies.
TDLR: jinsol is a false (only able to lie) and exchanged his ability with saegye. so now jinsol is a dare and he only said he loved saegye to take his ability. he used him and now saegye is a false.
I knew it....
It was my gut feeling from the start... It pisses me off..(╯°Д °)╯╧╧