Me again because I forgot my second request.

I know it sounds totally vague, but that's what I remember: the main character is a girl who belongs to an old family (clan?) of sorcerers. She summons a familiar or a guardian, who turns out to be a beautiful boy. (It's not Zero no Tsukaima because the boy is magical and mysterious). And then something goes wrong, like she wasn't supposed to have him?

I think there was a love triangle, too. Maybe. My memory gets vague at this point and I didn't finish it back then, but that's what I also can say:
1) it's not Ayahatori Shoukanjou
2) it's not Kaze no Stigma even though the old family of sorcerers is kinda similar
3) no one around knew about magic, etc, it wasn't a world where magic is everywhere, it seems it was actually our world?

Thanks in advance!

It sounds a like Her Majesty's Dog, although I don't think that's what you're looking for.

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Practically perfect in every way. - Mary Poppins

No, unfortunately, it's not that. It does sound similar but in the manga I'm trying to get the title of, the female character only met her "summon" at the beginning of the story. Everything was new for her.
In Her Majesty's Dog, they go way back.