Is it permissible to marry a 22-year-old girl without her guardian's consent if the guardian refuses due to a difference in nationality, knowing that the pillars of marriage are met and the goal of the marriage is to save the girl from ruin?

You have committed a grave error due to your relationship with that girl and your knowledge of her private matters, and you must repent for that. The wali (guardian) is a condition for the validity of marriage (nikah), but the wali does not have the right to prevent his ward from marrying a suitable match. Suitability (kafa'ah) is determined by religious commitment and character, not merely by a difference in nationality. If the girl wishes to marry you, let her convince her guardian. If he refuses, she has the right to refer the matter to the Sharia judge, who may order her guardian to marry her off or marry her himself. It is not permissible for you to run away with her to get married, and you must end your relationship with her and seek someone else.