(Attention! The following text includes spoilers up to the chapter Act of Love, and general lore around Yamato. It may affect your reading experience to read it before reaching a more grounded point in the story, and is meant as support to help the reader understand characters on a deeper level in case they could not do so by reading the main story.)
Yamato Tasumaki, although not visibly of Japanese ethnicity, his first name originates from Japan due to his family originating from there. His last name is not an official Japanese name, despite sounding so, as it is actually a mistranslation which happened as his family moved more into Central Europe, where language barriers and family deaths ended up with an incomplete name. He himself discovered this at a young age, as he researched out of boredom about his origins where he solved the riddle with incomplete information in less than a day. His core talent is hyper awareness, as he understood everything too fast at a young age, which made his intelligence grow exponentially ever since that "awaking", which felt more like ascension. Standing at 1.80 meters (5.9 ft.) tall and weighing about 80 kg (176 lbs.), he has a very macular build which reinforces his self perception as the peak human or Most Superior Human.
Yamato's self perception:
He himself rarely views himself in the same state of being as those around him, only when he directly engages with someone is he forced to "look down" to communicate, as he perceives it. His generalized, casual demeanor of superiority in every way is constant and part of his core personality traits. He is convinced that he is the best at everything, and even if someone seems to be better in a certain area, he believes he could surpass them if he tried. Or at the very least, if they would have started at the same time to train in that area, he would be better by now. This is another origin of his depression, as he views any and all actions as optional, as he can forese the outcome and actually acting often seems like a waste of time to him, since it would just be proving what he already knew anyway. He does not directly view himself as a god, much rather does he like to put himself even above that too. He does not want to be a god that coexist with other gods, as that implies a form of equality to him that he simply does not think exists. Godhood would be the closest, but still inferior, state a being can reach to compare to him. Yet any and all remain beneath him and that is one of the core beliefs he holds but never explicitly states. This ties directly into his behavior and treatment of those around him.
How he treats and acts around others:
Yamato acts with a sense of superiority that he usually hides behind a facade, where he fakes feelings like compassion and empathy with near perfection. He does this usually to gain trust and advantages over the other, not because he wants to be liked by them or fears isolation. He is incapable of casual life, as he will always try to manipulate those around him into thinking of him as special or superior. He does that mostly for his own amusement, but sometimes has a hidden agenda with their loyalty. One example for this would be Arina, where he only interacts with her for the sole purpose of seeing potential usefulness in her future. His manipulation can vary depending on the situation, and is not limited to a single area or set timeframe as his schemes can take multiple years, in which he visually perceives most outcomes and prepares for all mentally. His lack of empathy comes from a detachment which originates from a deep, nihilistic depression. He has a general disinterest in the world and existence itself in his original world, which is also a major factor in his detachment, as he does not view the world and anything or anyone in it as valuable or meaningful.
His view of Yuki:
Yuki is a central figure in his life, by far the most prominent and useful and most importantly loyal. In his previous isolation and lack of motivation, he was unable to find meaningful ways to interact with those around him. This changed when he died, which is how he met Yuki. At first, in the land between, he was fascinated by the existence of divinity but also disappointed in its, as he he already had assessed, inferiority to him. His feelings of superiority only grow when he beats Yuki in the chess game and psychologically outclasses her, by manipulating her into losing focus while remaining perfectly in control of his own demeanor. Originally believing she would not hold up her end of the deal, he was surprised when she actually fully submitted to him, though he understood quickly why that happened (see "The Psychology of Yuki"). This marked the beginning of their relationship, though he still saw her more as a tool or object, which she seemed to be fine with. Her advances into a more romantic and secure relationship were annoying him at first, but he did not take long to adjust to this to him unfamiliar level of emotional intensity. As their journey lead them to do harmful things toward others, he slowly began to realize how he develops fondness of her, not wanting the things they do to others to happen to her, which directly clashes with his nihilism. Her constant support and unconditional affection made him view her as an important part of himself, though still a possession, he would not share her or lose her at all costs. As his "most prized possession", he valued her imput and even found her antics adorable, a feeling he previously had not experienced before meeting her. She became something he would call desirable, in the sense that having her was a goal to strive for, thus keeping her would be a similarly logical goal to have. He of course is aware that he does not need much to keep her, as it is part of her nature to stay with him no matter what. Yet he finds himself wanting to do more than the bare minimum, which is what led up to the sexual actions taken in "Act of Love" and beyond. Though he did not initialize that jump in tone, he was aware of its implications and willingly allowed their relationship to escalate into an actual romance. Despite that, the emotion "love" remains missing in his heart, and he himself is unsure if that is just him failing to identify what it is, or if there is a genuine lack of it. But by now he is certain he is not feeling nothing toward her, which he considers an improvement in his ultimate goal to love her.
His goal and means to achieve it:
Yamato acts with two motivations in mind. One being the desire to dominate and control, providing his superiority and thus everyone else's inferiority, often out of sheer boredom and amusement. The other being to develop emotional attachments and feelings, which he believes will end his boredom and disappointment with life in general, giving him genuine happiness. Yuki is his biggest hope to achieving the latter, which is one of the main reasons he tries to mimic her advances to some extent, even though he struggles with the sheer intensity that is Yuki. His first goal usually leads to people getting hurt both physically and or emotionally. He does not mind any kind of "bad", as he finds the concept of good and bad laughable and beneath him, yet he understands how to categorize behaviors and people into each, which he usually uses to gaslight and guilt trip people into siding with him. The only real concern he has is children, as he has a fondness for innocence and kindness which he connects with purity and uniqueness, as it cannot be artificially made according to him. Himself being a bad person, does not stop him from his fascination with the concept of kindness and purity as he views it from an objective lens and expects others to do so as well. He believes that artificial purity is a paradox, as purity in itself needs to come naturally, otherwise it is just pretending. Even though he has this fascination with purity, his romantic and sexual preferences align more with Yuki's character as he does not see anything wrong in her behavior or personality, except the occasional overdoing on her part, which he does not consider a serious problem as he believes he can take care of any complications that might arise from her actions. He usually analyzes everything extremely close and detailed, his hyper awareness making this only take fractions of a second. When he experiences new things, he saves them up subconsciously because he has a need for preparation that often includes thinking about minor details that could lead up to greater reactions, which he usually does on the side simultaneously to his everyday life.
His Hyper awareness as separation:
His hyper awareness is what originally caused his feelings of superiority to form, as he learned that no one else seemed to view and comprehend what he could. The fact that people forget little things, while he could just repeat an entire day from years ago with every detail of every room he was in within a couple of minutes made it frustrating to converse with others, which eventually lead to him shutting down into the previously mentioned nihilistic state. This caused a feeling of being misplaced, and he interpreted this place he was wrongfully put in as above everything.