how did mew mew getr a shadow crystal

Mad Mew Mew gets the Shadow Crystal by defeating her as the secret boss after using the required key item to reach her fight. The publicly available guides I found tie that reward to the optional Chapter 5 secret-boss route, not the main story path.

How it happens

• Collect 10 Pink Coins.
• Buy the Mystery Key from the cliffs shop.
• Use the key on the pink door.
• Beat Pink / Mad Mew Mew to receive the Shadow Crystal.

Why people are confused

Some guides treat “Mew Mew” and “Mad Mew Mew” as the same secret-boss thread, while others note that Chapter 5 claims are still being sorted out and should be treated carefully. The older Deltarune lore also says Shadow Crystals come from defeating special secret bosses and can be given to Seam.

Quick note

If you meant the original Undertale/Deltarune-era Mad Mew Mew , her Shadow Crystal is linked to being defeated as a secret boss rather than a normal battle.