After some try i got Mian true ending, but still can't unlock her birthday event to complete her profile.
The japanese wiki at http://www43.atwiki.jp/dclubzero/ states that i've to get a mail from Yuki, and she will reveal Mian birthday but i don't understand how, should i just spend some time with Yuki, or get her in ETS and to follow her "route"?
Seems like i got my answer.
To unlock Mian birthday event you've to first meet her, then meet Secchan and choose her again for the next 2 weeks (3 weeks total), then raise Mian affection doing ETS events, then when Mian is away (9/30) choose for the 4th time Secchan, if everything went well she'll invite you out, after choose only Mian, 10/16 you'll receive a mail from Secchan with Main birth date.
To unlock her birthday event you must:
1st have a date with Setsu (choose her at least 3 times in a row, then 1 other time to get her date)
2nd unlock Mian key item (cat apron)
3rd have enough money to buy it before her birthday
atm i don't know if buying her birthstone is essential, maybe as a present for her birthday, i'll try it again in next playthrough.
Saix, he is a member of organization 13. S-A-I-X, got it memorized?