ML did reveal she was a mermaid in the last timeline, which angered MC, so he didn't want to make that mistake again, and MC doesn't want to reveal she's a mermaid because she is trying to cut all ties with her biological family, by making them think she is already dead. In the last timeline where it was revealed, her family guilt tripped her and forced her under their control. ML is trying to prevent that in this lifetime, because ML went back in time and is fixing his errors
Hmm, technically both of them. MC said she took the regret pill 8 times to try and save her friend (ML, but MC don't know it's ML). This timeline though ML went back before he met MC, so MC doesn't know about it. Both of them are fricking with time, but MC doesn't know ML did, because ML went back further, or with a different method. The regret pills only let you change 1 hour of the past, not go all the way back and relive every moment, from my understanding so far.
Just get marry problem solved (╬ ̄皿 ̄)凸