Is it permissible to marry after repenting from a relationship that resulted in pregnancy, and is the child considered legitimate in this case?

You have done well by repenting. Be wary of being lenient in illicit relationships and forbidden seclusion (khalwa). Scholars have differed regarding the ruling on a fornicator marrying the woman with whom he fornicated before her womb is cleared (istibra’). It is more prudent not to marry her until she delivers her child. The lineage of the child is not attributed to the fornicator; rather, it is attributed to the mother, unless the fornicator claims him (istilhaq) according to some jurists, but our fatwa is contrary to that.