When Meroko and Takuto first appear to Mitsuki, they're extremely surprised that Mitsuki can see them since humans aren't supposed to be able to see Shinigami.
Is it ever explained why Mitsuki can see them?
When Meroko and Takuto first appear to Mitsuki, they're extremely surprised that Mitsuki can see them since humans aren't supposed to be able to see Shinigami.
Is it ever explained why Mitsuki can see them?
It's because:
The soul of Eichi has been watching over her since he died. When Sheldan tries to take Mitsuki's soul, it's explained why Eichi is the reason why Mitsuki can see Shinigami. This is what saved Mitsuki and Takuto after they get pierced by the Mystere's scythe at the end of the series. At the end of the manga it's revealed that Eichi refused to allow Takuto to take his soul (this was Takuto's first assignment as a Shinigami) because he wanted to meet Mitsuki again (because of the promise they made to meet again when they achieved their dreams) and stays with her for 6 years until the end of the series, when he see that she is now happy and wants to live.