If you're asking about 'Brook' from One Piece, then yes, in a peculiar way he is. But remember, immortality in his case comes with a rather unique twist. Brook ate the Yomi Yomi no Mi or Revive-Revive Fruit, which gives its consumer a second life after they die. The interesting bit is that when Brook initially died, his soul wandered around unable to find his body until it had become a skeleton.
But his DF ability then kicked in and voila, he got his second life in the form of a living, walking and even talking...skeleton! So technically, unless his bones are destroyed, he can keep on living. But don't forget, in real world scenarios, it's just a conceptual, fiction-based immortality.
So, it's a yes with a very heartily laughter, all because of our beloved skeleton's sense of humor. Don't you just love the way the writer played around with the concept of life and death? It's a testament to the creativity One Piece has been praised for since it first came about.
Brook is not completely immortal in "One Piece". Brook ate the Yomi Yomi no Mi, which permitted him to come back to life after death. However, this does not render him invincible or immune to all sorts of death. If he dies again after using this second life, he will remain dead. Also, he can still be hurt and face threats that could kill him, therefore he is not eternal in the classic sense.