Consider that as a plot hole. Because in ch.5 the merman explains that he lost the ability to make people immortal?
The racoon lady explained it as the spell failed. She thinks it might be due to the spell being made for those of the opposite gender which he is not. So in that case he can't become fully immortal and younger.
The spell didn't work because of the gender of the merman partner seems quite right, but one more question, they said a lot of meman makes their partner immortal and died when they have offspring right? But how about the partner? The plot said merman/mermaid have a child so they can die, but theres no mention about their immortal partner, so i guess theres gonna be a lot of immortal widow right? Since i assume in the past theres a lot of mermaids and merman but they all died and our merman mc was the last merman.
Even thought they didn't explain it all that much I think the moment a child is born, they both loose their immortality. The time the uke found the merman's body, he also found his wife's. Also the raccoon lady had the skull of a merman and human which the letter said was the child's " predecessors". So I assume the other head was the wife. She also said that the mermaids are similar to those worm thingies. The worms are immortal untill they give birth and then they both start to age away and die. So even though they didn't say anything about the bride I assume she too goes back to aging and they die together.
the spell didn't worked properly so he just stayed in his original from(his form when he started digesting the merman's body)
So, im confuse isn't when a human eat mermaid flesh they become immortal and will back to their youth? Why he still looks like in his 50th? Or the merman only use a spell to make him immortal not by eating his flesh, but in the panel when the merman saving kiyo for the first time, we can see he try to feed him his flesh (see the bandage in the end of his tail) so why kiyo didnt get younger? The thing is, he immortal, but immortal in his 50 y/o body, isn't it hard for him? 50 yo body not as strong and healthy like their young days,
but what a nice read, kinda sad he is the last merman, wish theres another merman/mermaid that have the same way of living like him, so we can see the others, and interesting mermaid design, its mixed between a dragon and a fish.