User blog comment:Koishi-Komeiji/Kyouko Sakura/@comment-25848535-20190810153142/@comment-24965192-20190810160236

Yep! In the way this game is made, you do need an NP5 to reach a 300% base NP gauge to unleash a Doppel. The damage for a Doppel will always be 750% (for AoE) or 1800% (for ST), following the 1.0x card damage multiplier because the Doppel's card type is Typeless. You also cannot chain Buster, Arts, nor Quick with a Doppel card. And yes, the OCE is consistent regardless of current charge.

In Magireco, Doppels are unleashed when you've maxed everything you can on a Magical Girl (except their CE slots) and then reach 200% NP gauge in a battle. They're an extremely stronger version of their Magia (Noble Phantasm).