Kerberos

Kerberos (voiced by Katsumi Shiono) is an artificial Undead born from the card Hiroshi Tennoji made from the DNA of all 53 Undead, surpassing the Black Joker itself, Kerberos was created as the fifth Ace that Tennoji intended to be the victor of the Battle Fight he set up. Unlike the previous creations, Kerberos was beyond Tennoji's control, killing his entire staff upon being born from the Stone of Sealing. The Undead succeeded in absorbing the decks of Chalice and Leangle, along with Jack and Queen cards of Garren. Kerberos even overpowered Blade King Form at first, but was defeated by Royal Straight Flush.

But Tennoji sealed the Undead himself into the Change Kerberos card for his true plan: Using a surgically graft device based on the Rider System technology to fuse with his creation, becoming Kerberos II with Tennoji's face on the monster's chest. It took all four Riders to fight Tennoji/Kerberos II, with Blade King Form's Royal Straight Flush and Wild Chalice's Wild Cyclone to negate the fusion. However, the Giraffa Undead kills Tennoji and takes the Change Kerberos for himself in the fight's aftermath for his own agenda.

Giraffa tried using the card's latent ability to seal other Undead against Chalice, but Blade jumped in the way and it absorbed all of the Spade Suit cards instead. In a later battle, he tried stealing Garren's cards the same way, but Garren pulls him off a rocky shore cliff and seals him. Kerberos's card isn't directly seen being retrieved in the collection of cards that floats to shore, but it presumably still is.

In an alternate future, the Change Kerberos was used as the Ace card for Kamen Rider Glaive, with two copies of the card used in creating Riders Lance and L'arc.

In an alternate timeline where Shocker conquered the world thanks to the Shocker Greeed, Kerberos represented the Undead in a coalition of the various evil organizations and monster races which were allied with Shocker. With the Giraffa Undead by his side, Kerberos was present at the conference of alliance.

Trivia

 * TBA