I understand the ruling on marrying prepubescent girls is that you require the permission of the wali, however I am curious in regards to what the ruling is on marrying a prepubescent girl who does not have a wali, or even a prepubescent girl who’s wali is not a muslim, would it be permissible to marry them without anyones permission purely from a fiqhi perspective?

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Such a marriage is only valid with the permissible of a wali AND if this marriage theoretically is in the interest of the girl and does not cause her any harm. Otherwise it would not be valid. If a girl has no wali (such has her father/grandfather has passed away), the jurist (qualified Mujtahid) would be her wali.