I managed to get a secret unnamed 2 star character on the second mission: "I don't want to be Santa -Intermediate-", the one that takes 20 stamina.
Here are a couple of screenshots:



Keep in mind, that since she's a 2 star you will have to manually lock her afterwards, if you manage to find her.
So far I've gotten her twice, both when replaying the second mission, I did not get her through the first play-through, so she might be available through the other missions.
I also noticed that the similar skill upgrade shows up on manyu, dragons and other blooming items, but so far I've only experienced a skill upgrade through the second instance of the character I've found, so I'm not sure if this means anything.
She also shows up in the gifts section, but her gift page doesn't appear, and she also doesn't have a page under collections.