COLUMBINA IS STOKED FOR NEW FRIEND. Since LA was so competitive she knows shit can get nasty, and whether she and Mami can ACTUALLY be friends it was good to get her close as early as possible. I mean she's assuming they can be real friends, but just in case, y'know? But the fact that Mami was naive of a couple key facts made Columbina kind of instantly protective. Even though she's grumbly right now, in fact she's pretty good at denial, and having someone who flat out doesn't know the truth makes just an excellent help with that :D. So she'll be doing her best to keep the nature of Witches (and any other unpleasant truths about Being Meguka) from Mami. It's better for Mami and it's better for Columbina everyone wins.

If we get more Puella, she might decide they are a team and that Mami is the leader. Yes, especially if she's lying through her teeth at her constantly. Obviously Mami is the cool, collected one. Unless they get a more shounen active Meguka, yeah. Mami should be the cool obviously competent leader.

SO HEY WHAT IS MAMI AND KYOKO'S BACKSTORY ANYWAY