Here's how Deadfire's reactive component works:
-Stage 1 (Default look): Start of match
-Stage 2 (Clothing and hair color change): Occurs when you deal 400 damage to opponents/Cube Monsters OR when 40 players remain in the match (whichever comes first).
-Stage 3 (Green flames): Occurs when you deal 1,000 damage to opponents/Cube Monsters OR when 10 players remain in the match (whichever comes first).
These same rules & thresholds apply to the Pickaxe and Back Bling as well.
Basically, everyone can transform to the final stage of these items by surviving in the match (regardless of your playstyle), however you can transform quicker by engaging with cube monsters or other players earlier in the match.