Death (死神, Shinigami, lit. "The Grim Reaper") is present in all Castlevania games, except for Haunted Castle, Castlevania: The Adventure and Castlevania II: Belmont's Revenge. Death is often a boss fought toward the end of the game. He is considered Dracula's second-in-command and most trusted minion. Sometimes he has played an important role to the storyline, such as Castlevania: Lament of Innocence. He is a playable character as the skeletal knight in Castlevania Judgment.
In most of his appearances, Death closely resembles the cultural personification of the Grim Reaper, being a legless skeleton in a hooded cloak with a scythe. Like Dracula, but in a lot fewer games, he can change into a second form upon being defeated in the game.
He is voiced by Dennis Falt in Symphony of the Night, Patrick Seitz in The Dracula X Chronicles and Order of Ecclesia, Douglas Rye in Judgment and Tom Wyner in earlier games. Death also appears in the 2018 crossover fighting game Super Smash Bros. Ultimate as a stage character.
The character of Zobek, Lord of the Dead (voiced by Patrick Stewart) in the reboot Castlevania: Lords of Shadow has been speculated to be the version of Death of this universe, due to his relationship with Dracula. Additionally, the Lord of the Necromancers is at one point referred to as "Death itself". In the game's sequel, when Gabriel Belmont is forced into a confrontation with Zobek sheds his human form and takes on a more traditionalized appearance of Death (including wielding a large scythe).
Source: Wikipedia, "[en.wikipedia.org/wiki/List_of_Castlevania_characters%23Castlevania_and_Simon%27s_Quest List of Castlevania characters]", available under the CC-BY-SA License.
Zead (ゼアド Zeado?) is a main villain who works with Isaac in Castlevania: Curse of Darkness. A priest who claims to seek the end of the curse and gives Hector clues as to Isaac's whereabouts.
He reveals that he actually is Death in disguise and engages Hector in battle, although he is ultimately defeated, surprised to have been defeated for the first time by a mere human.