I would just like to know what Islam says about marriage between different races? Is it permissible to marry a revert, who has good character even though the girl's Wali is opposed to it?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Islam does not permit discrimination between people based upon their nationality or color. So, every practicing Muslim with good character is suitable to be married. But, according to the majority of Muslim scholars, executing a marriage contract without the permission of the guardian of a girl is not permissible, and this is the preponderant opinion.

Most probably, a woman needs a guardian to help her choose a good husband. So, a woman should not marry against her guardian's will.

However, if he prevents her from marriage without any legal reason, then a judge in an Islamic court may marry her to someone to prevent injustice by her guardian.

Allah knows best.