What is the ruling on a son's desire to marry a girl who suffers from psychological and behavioral problems, as he sees it as a form of protecting her and a matter of trust, while his parents strongly object out of fear for him and the future of his family, so is this marriage obligatory for him or not?

You have done well by showing concern for the girl, caring for her, and being keen on teaching her matters of her religion. Perhaps it would be a completion of your kindness to her if you agree to your son marrying her, provided she is religious and of good character. This requires the permission of her guardian or a duly authenticated legal power of attorney. However, if she is not satisfactory in terms of religion and character, then marriage to her is not advisable. Try to persuade him by all means possible, and seek help from a suitable person he trusts to convince him of the necessity of obeying his parents, and that his vision does not entail a ruling.

Seek help through prayer to Allah, for the supplication of a parent is answered, as Abu Dawud narrated from Abu Hurairah that the Messenger of Allah, peace and blessings be upon him, said: "Three supplications are answered without a doubt: the supplication of the parent, the supplication of the traveler, and the supplication of the oppressed."

We draw attention to two matters: avoid leniency in mixing (between genders), be cautious in the future, and residing in lands of disbelief is not permissible except out of necessity or need.