Yi sang is a particularly quiet, mellow man. At the beginning of the game, he fails to stand out much among his fellow sinners due to his tendency to stay silent and watch rather than actively contribute to discussions. On top of this, when he speaks, his words are often obscure and riddle-like whilst also delivered monotone. His expression never shifts much either.
Nevertheless, Yi Sang does not come off as unkind. He appears to care for the people around him in his own way. Similarly, for the most part the Sinners seem to accept him as simply being a peculiar person, rather than interpreting his attitude negatively or getting frustrated with him. In Intervallo I: Hell's Chicken, betraying his aloof and inexpressive appearance, he's shown to be rather eccentric and sentimental, forming an immediate attachment to a burnt potato sprouting flowers to the point of being in visible distress over being separated from it by Don Quixote at Meursault's order.
Come Canto IV: The Unchanging, and more is revealed about Yi Sang little by little, such as his concerningly low level of self-esteem and outward disgust at Dongrang's callous behavior concerning the many deaths of his fellow K Corp. employees and the suffering they are consistently put through. Though for a large part of the story he refuses to open up to Dante and The Sinners, after being questioned by Dante in regards to his emotional state after being confronted with his old companions, Yi Sang reveals that he considered the day he met Dante and gained immortality against his will his greatest moment of despair, quietly wishing for his own eternal rest. This trend of suicidal depression is echoed by Dongbaek as she justifies her attempted murder of him as something he desired, which he does not retort. Following this interaction, Dante resonates with Yi Sang's mental state, being able to see Yi Sang with a large, cracked void in his chest with no signs of mending.
Within the dungeon, the source of this hopelessness is finally revealed: being severely affected by the loss of the League of Nine Littérateurs, a place in which he found comfort and safety amidst the harsh, chaotic world he lived in as well as people who supported him and who he in turn wished to connect with and bring happiness to, and ultimately being manipulated and betrayed by the last companion who remained, he was left grief-striken and without hope for life, alone in the world. Affected by the abuse he suffered under Gubo, he believed himself to have no free will or a say in what happened to him, wallowing in despair and going through the motions until eventually finding himself under Limbus Company, given the vague premise of finding hope again and regaining his "wings", the sense of freedom and endless possibilities he felt back when he was with the League of Nine.
Once come to terms with his own past and a newly found sense of hope encouraged by Sang Yi, an alternate version of himself living within his personal mirror Yeonsim, as well as the last remaining will of Dongbaek, he becomes much warmer and open towards Dante and his fellow Sinners, wishing to help them and not watch the tragedy of the League repeat. This can be most seen in Intervallo II: S.E.A, where he actively tries to keep the group in good spirits however he can and help Heathcliff and Ishmael reconcile, and often showing his own appreciation and compassion for his companions and those who aid them along the way such as Pilot.
Though at the same time, despite his position as a genius he's shown to be rather socially awkward and naive; often being distracted by things he finds intriguing, not understanding jabs and often taking things literally, such as in Intervallo III: Yield My Flesh To Claim Their Bones where he misinterprets Rodion and Gregor's attempt to avoid a complicated lecture by him and Faust when they ask about Nest T's architecture and getting carried away explaining it due to his own interest in the topic. Another example appears in Canto VI: The Heartbreaking where he's overwhelmed by Ishmael and Hong Lu's joking insistance that he would find it odd that Heathcliff would recieve a love letter, clarifying that he does not believe that and did not intend to imply so or insult Heathcliff, which Ishmael explains to him immediately after to be mere jest, as well as being impressed by Rodion deducing his favorite color and giving a description that fits his personality and behavior based on his choice.
During the Canto, Yi Sang is shown to be highly unsettled at the utilization of his technology and research in the human dough experimentation conducted by N Corp. in collaboration with Aseah, his former compatriot, as well as the use of his mirror in Hermann's plans to manipulate Nelly and Catherine into the creation of the laboratory, believing the resulting experiments to be too high a price for any single human life.
