Liam can be either kind of murderous, depending on the route he takes. In pacifist he is very kind and makes friends with everyone, however it is more challenging than in Undertale, especially with Asgore because he never backs down and even when he returns to his normal form after the multiple battles you have with him. In genocide Liam is obviously evil and tries to murder everyone, he succeeds just like Frisk/Chara in Undertale.
Liam has black hair and green eyes, he wears a squared shirt that is black and white. Because he is a human most monsters excluding Chara, Asriel, Asgore, Undyne and Burgerpants often mistake him for a walking, talking chessboard. In genocide Liam's appearance completely changes his face looks more sadistic and murderous. His face will melt without warning but then reform. Liam's shirt has also changed it has dust covering most of it and when his LV increases his eyes glow bright red for a split second.