What is the ruling on completing an engagement with a girl for whom the Istikhara prayer was performed, but thereafter undesirable traits became apparent, while the suitor has feelings of love and is uncertain of her feelings?

If there is a deficiency in a woman's religious commitment or character, it is better to turn away from her. However, if she is religiously committed and has good character, then you should perform Istikhara (prayer for guidance), and what Allah chooses is always the best. If marriage to her is not facilitated, then seek someone else, and continue to supplicate and ask Allah to ease your marriage to a righteous woman.