Fang Yuan's current personality developed over the course of 500 years of his first life in the Gu World, as he lived among ordinary mortals, Gu Masters and Gu Immortals, which gave him deep wisdom and understanding on the various aspects of life. He has a complex personality with a breadth and depth of opinions as well as a mindset vastly different from his peers.

He is a man of endless focus and perseverance. Despite being more powerful than his enemies many times, he never underestimates them and remains eternally vigilant when it comes to planning and executing his schemes, always calculating his chances realistically, expecting failures to happen and making contingencies for those possible failures.
Fang Yuan truly only counts on himself and never fully relies on others when it matters, as he sees only his own strength as the greatest and most reliable assurance.
He is a true demonic cultivator without moral restraints and his actions reflect that of a true opportunist, therefore, he is able to betray and sacrifice anyone at will, regardless of their supposed relationship, as long as this action benefits him and yields a greater gain. Compared to others influenced by compassion to not take action against weaker opponents, Fang Yuan is more than willing to take them on, only if the enemy is beyond Fang Yuan's strength or ability would he mindfully retreats; which can be summed up with one of his mottos: 'Bully the weak and fear the strong'.
To Fang Yuan, coldness, arrogance, humility, conceit, all of these are masks, they are tools for him to obtain profits. But because of this trait, he also causes many to distrust and be wary of him.
Fang Yuan shows most of his brilliance in scheming against his enemies by using tactics instead of brute strength and is proficient at finding out his enemy's weak points and making use of it to its fullest potential.
After experiencing many things in his 500 years of life, he came to the conclusion that everything was worthless in front of eternal life. Unlike others who are consumed with fame, glory, beauty, repute, worldly riches, and (intimate) relationships, Fang Yuan has abandoned those ephemeral pursuits in favor of gaining immortality— even if it means dying.
While utterly merciless to opponents or people he sacrifices for his goals and ambitions, in daily life, Fang Yuan is reserved and polite. As he doesn't care about face or status and is indifferent to the opinions of others towards him, he generally treats the random people he might meet in his travels politely as if they were friends. Further, he never takes things personally and has no issues forgiving minor slights or mistakes, particularly if the other party bears him no ill-will. He once stopped a mortal father from severely beating his daughter (as he was afraid for her safety and believed a Gu master would severely injure or kill her if she were not punished) for bumping into Fang Yuan (who had high status as a Gu Master) and Fang Yuan told her it was a small matter and he didn't mind.
Fang Yuan's character has deep similarities to the concept of the 'Machiavellian' who is a person so focused on their own interests they will manipulate, deceive, and exploit others to achieve their goals. He cares about nothing but his own interests. To him there are no permanent allies nor permanent enemies. There are only permanent benefits. Even in the face of total failure he stands unfazed and calm. Even having a smile as he dies, because he lived true to himself even as a true demon. He is neutral towards the world and life itself, doing whatever necessary for his single wish. Only when following rules that benefit him will he then abides. If disregarding the rules will benefits him he does so without hesitation. Fang yuan does a cost-benefit analysis every time he makes a decision, he'll do "righteous" things if they benefit him more and "evil" things if they benefit him more. He feels absolutely no sense of obligation to allies or people who help him.
Fang Yuan doesn't have any sort of urge to treat people cruelly if there's no reason. He simply will disregard anything that isn't helpful to his goal of immortality. The relationship with his brother from the beginning was like that; to Fang Yuan, he was not of any value whatsoever and he planned to ditch the village in the future, so he disregarded and ignored him. He didn't bother being friendly since he saw no value. He also is able to seamlessly act as a righteous path member whenever it suits him. The only thing that matters is whether conforming to their behavior is of value for his goal.
He often mentioned that he does not believe in eternal enemies, only in eternal benefits and as such would trade and form alliances with (previous) enemies if it benefits him.