Wizkids Rules!

Since Doomsday's ability chooses a character die (not the card), this raises a couple questions.

Doomsday: Unimaginable Bloodlust
At the beginning of the game, choose a character die's name.
When fielded, KO target character die with the chosen name.

Can a player choose the a specific type of token character? (Such as Sentinels produced by Master Mold or Plants from Poison Ivy)

Could a player choose Sidekick Character Dice?

No to both questions.

A token is a component generated by an ability, not a Character die. There are circumstances when an ability affecting a Character die can include tokens. However, the application of Doomsday’s ability is not one of these circumstances.

At the beginning of the game, there are no Sidekick Character dice, only Sidekick dice – a Sidekick die becomes a Sidekick Character die when it is in a rolled game state and the die is showing a character face.