Aventurine Of Stratagems can easily be one ... This also makes it important to find the characters that are good without any Eidolons, since it's nearly impossible for an F2P player to roll ...

If you haven't pulled Aventurine, though ... Express Gift allows you to pick up one of their Eidolons for free. As with the rest of the possible choices, you'll only want to pick between Bronya ...