Who will be the wali for an ex Christian who wishes to marry?

What if some brother wants to marry this sister and the wali says no on the grounds that he is unsuitable for her?

If she does not have any relative who is a Muslim then the Imaam will be her wali.[1]

What applies to the father as the wali is the same thing that will apply to the Imaam as the wali. The consent of the wali is very important. It is essential not to just jump from one wali to the next without a valid Shari’ah reason.

Reference:

[1] It was narrated that: Aisha and Ibn Abbas said: “The Messenger (ﷺ) of Allah said: 'There is no marriage except with a guardian.' ”According to the Hadith of Aishah: “And the ruler is the guardian of the one who does not have a guardian. ( Sunan Ibn Majah, Hadeeth: 1880, graded as Sahih (Darusalaam))