Takumi Kusakabe is Maron Kusakabe's Father.
He along with Maron's mother Koron appear in last episode shortly.
Unlike in anime where we see them shortly, in manga he remarried Koron and came back to Maron.
In chapter 20 of the manga when Finn Fish turns into fallen angel, she tells Maron that her parents have been controlled by the Demon Lord past 17 years.