Mo Xuanyu is noted by Wei Wuxian to have quite an attractive face, though standing a few centimeters shorter than Wei Wuxian did in his own body. He often painted his face white and rouged his cheeks, giving him the appearance of a "hanged ghost".
In Animation, Mo Xuanyu looks similar to Wei Wuxian. He has black hair that is in a wild ponytail and gray eyes. He wears black robes.
In the Manhua, Mo Xuanyu again looks similar to Wei Wuxian. His first moment summoning Wei Wuxian shows him with bronze slitted eyes. His hair is black and pulled into a black ponytail. His robes are black.
Described as a lunatic by everyone who knew him, it's unclear if this came about for his homosexuality, his interest in demonic cultivation, or the eventual mental break he suffered at the hands of his abusive family following his expulsion from the Jin Sect. Mo Xuanyu did have a vengeful side as he was willing to sacrifice his soul to summon a demon that would kill his family.
Mo Xuanyu was an incredibly tortured soul, piled with the expectations of his servant mother to gain the recognition of his father, Jin Guangshan - the then leader of the Jin Sect. A "cutsleeve" (homosexual), he was sent to the Jin Sect to learn cultivation but was cast out in shame after harassing his half-brother, Jin Guangyao.
He returned to Mo Village and his mother died soon after, leaving him in the care of his abusive aunt and her son, Mo Ziyuan. Both were jealous and resentful of his wasted opportunity at becoming a cultivator of a prolific sect, and his cousin took to stealing Mo Xuanyu's artifacts and talismans, physically beating and verbally abusing him until such a point as he become mad.
In his distress, he sacrificed his soul to summon the Yiling Patriarch, Wei Wuxian, into his body and take revenge on the Mo clan for their abuse of him.
Mo Xuanyu used to be in love with Jin Guangyao, who was his half-brother.
Madam Mo was unable to stand her younger sister, who was the daughter of a servant. She also had the same mean behavior towards Mo Xuanyu, her nephew. Her resentment towards her sister and nephew only increased after Mo Xuanyu was taken away by his father, Sect Leader Jin Guangshan to cultivate, but her son was refused.
Mo Xuanyu had an antagonistic relationship with Mo Ziyuan. Mo Ziyuan would steal Mo Xuanyu's talismans and cultivation items, getting away with it due to Madam Mo.
Mo Xuanyu believed Wei Wuxian to be an evil spirit that was capable of taking revenge on the Mo family, and sacrificed his soul to summon him.
Jin Ling is Mo Xuanyu's nephew. Jin Ling viewed Mo Xuanyu as a lunatic and had a negative opinion of him. He did not know that Mo Xuanyu gave up his body for Wei Wuxian.
Abilities & Weapons
Mo Xuanyu is noted to have a much lower level of cultivation than Wei Wuxian did at the time of his death. This isn't surprising, as he never got much opportunity to cultivate before being cast out, but Wei Wuxian's golden core and spiritual strength did not follow into Mo Xuanyu's body, and he is left with the spiritual and physical strengths that Mo Xuanyu possessed before offering his body to him.
He had some knowledge of demonic cultivation, given the array and ritual he was able to perform to summon Wei Wuxian, but his golden core had not formed and his body was not as sturdy as Wei Wuxian's. As such, he was only capable of demonic cultivation, and not any of the paths the Jin Sect would have taught him had he stayed and learned from them. Not having any weapons and all his tools stolen by his cousin, he used his own blood to draw the array which summoned Wei Wuxian, and painted it with his hands.