You can choose any Magic character that has ever appeared in any official product or story, including characters that have appeared only in art. You can even choose characters who don’t have official names, as long as they’re clearly defined characters, like the Wanderer.

Vorthos’s ability applies to any spell in which the chosen character’s name appears in any form, whether that’s possessive (e.g. Teferi and Teferi’s Protection), or a related word (e.g. Avacyn and Avacynian Priest), or a word that’s derived from their name (e.g. Niv-Mizzet and Izzet Charm).

If a character has gone by different names, you may count all versions of that character. For example, choosing Lim-Dûl will offer you a discount on (SPOILERS) The Raven Man. Choosing Urza will offer you a discount on (SPOILERS BUT VERY OLD ONES) Blind Seer.

As with other acorn cards, Vorthos cares only about the actual printed card you’re casting to determine whether the chosen character appears. But would Vorthos want it any other way?

Vorthos’s ability cares whether the chosen character appears in the name, flavor text, or art of the card. It doesn’t care about rules text, watermark, set symbol, artist, and so on. No, choosing Maro as your character doesn’t let you get a discount on Look at Me, I’m the DCI.

Unlike most abilities that reduce the costs of spells, Vorthos’s ability cannot be applied to generic mana costs. For example, a spell that costs {2}{B}{G} to cast would instead cost {2} to cast. One that costs {4}{U}{U} would instead cost {4}{U}.