Alexander Gregory, from the early days of the game
If a card with no mana cost gains flashback, it has no flashback cost
Honorable Mention: Incubator Tokens The Incubator tokens from March of the Machine featured on some of the strongest cards from 2023 (Chrome Host Seedshark, Sunfall), to the point where they almost overshadowed the big splashy battle mechanic from the same set
Final Fantasy 13 is also this sets only instance of the Meld mechanic, which lets you fuse the Fang and Vanille cards into Ragnarok
On top of that, it synergizes exceptionally well with Ninja of the Deep Hours, as it packs evasion, and since you can return it to your hand, you can keep scrying multiple times throughout the game
If a cards health falls below zero it must be discarded, with damage carrying over between rounds