The Category Ace Undead are the best fighters out of the Undead, with Chalice as the strongest followed by the Spider Undead. Their cards are an important part in the Rider system, with the Riders donning armor modeled after the animals the Aces represent. All Aces appear in the series except the Stag Undead.
The Spider Undead shoots webs from his mouth to entrap his victims, sucking out their lifeforce. He is an accomplice of the Peacock Undead, appearing for Garren to seal while he spewed out golden spiders to latch on ideal hosts. Once finding an ideal host in Mutsuki Kamijo, the Spider Undead intentionally had himself sealed into the Change Spider card to complete the Leangle. Though sealed, the Spider usually takes over Mutsuki's body, using him to become more powerful for the Battle Fight until he no longer needs a human host. Affected by Titan's venom, Spider starts to completely take over Mutsuki to the point of turning into the Undead himself. The Spider was forced out from Leangle when attempting to assume king form, with Mutsuki free of the Undead's influence thanks to Hikaru Jo giving Noboru Shima a stronger influence. The Spider was properly resealed by Mutsuki with the King Rouser. Though unsealed in Missing Ace by the Albino Joker, the Spider Undead was resealed by J-Garren.
Episodes 11-14, 42, Missing Ace
Given the name "Chalice the Legendary", the Mantis Undead is the best fighter of all the Undead. As such, Hajime uses the Undead's form and assumes the identity of "Kamen Rider Chalice" while suppressing his Joker nature. The form of the Undead is how the Rider form appears save with an Undead belt, as noted by the Eagle Undead before Kamen Rider Blade sealed him. Though Leangle unseals the Mantis Undead, he is resealed by the Black Joker.
The Beetle Undead is very strong and caused much trouble for most of the Riders until Leangle reseals him with the Polar Blizzard.
An artificial Undead born from the card 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. Keroberos was beyond Tennoji's control, killing his entire staff upon being born from the Stone of Sealing. The Undead absorbed 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 4 Riders to fight Tennoji/ Keroberos II, with Blade King Form's Royal Straight Flush and Wild Chalice's Wild Cyclone to stop the fusion. However, the Giraffa Undead takes the Change Keroberos for himself in the fight's aftermath for his own agenda. Though thought to have been lost in the fight with Giraffa, the Change Keroberos was used as the Ace card for Kamen Rider Glaive in Missing Ace, with 2 copies of the card used in creating Riders Lance and L'arc.