It's a cute pet. You can buy it on Mamazon.mom. The duck encourages you to a point where your "energy" (needed to challenge dungeons) restores immediately.
Restoring Duck After Restarting Game Edit
- Restore Purchases Option - Main Title Screen: Red Flower --> Restore items. In Yamada's room, you can tap Yamada at his computer and then tap Options from that menu to get to the same place.
- Buy Another, Get Refund - I bought another from the market. Then request for refund for my previously bought duck. Google refunded me within 1sec.