From Dungeon Defenders 2 Wiki
Defender Packs provide a way to get costumes without purchasing premium currency. This allows Free to Play players to customize their characters.
The Defender packs are earned through the Onslaught Game mode that released on December 12th 2017. These Defender Packs can be re-rolled with defender medals for 100-12,000 defender medals depending on how many times you reroll them.
When a Defender Pack is opened you can get any cosmetic, skin, tower skin, pet consumables, or pets that is currently obtainable in the game.
The only flairs that can't be obtained are the mastery exclusive flairs, and the onslaught floor exclusive flairs.