I dislike lies

SIN-YOU

"The appearance of the Sin-you is similar to that of a Qilin, but more feral and imposing. It is a large quadruped with a feline or ovine body, a shaggy mane, and is either depicted with hooves or feline paws (the latter often to stress its difference from the Qilin). It has a single, unbranching horn in the center of its head, like a western unicorn. The Sin-you’s eyes are said to be very intense and imposing, figuratively burning into whomever it gazes at in a predatory fashion."

POWERS

"The Sin-you is highly symbolic of justice, and is believed to have the power to know if a person is lying or know if they are guilty with a glance. It sometimes depicted at court beside the the ruler or judge: if a person told a falsehood in its presence, it would leap forward and impale the perjurer though the heart with its horn. In other instances, the judge would put convicted murderers before the Sin-you, who would slay them in the same fashion if they were truly the perpetrator, but leave the innocent unharmed."

IMPORTANT: He can sense lies/falsehoods from humans only. Non-humans, such as animals, demons, other supernatural beings and so forth are immune.

SIN-YOU FORM ONLY
+ Can always tell when someone's lying or not.
+ However, if in sin-you form, Li would feel compelled to impale the liar or "perjurer" with his horn. This would be why he try to keep his distance whenever he's in his other form.

HUMAN FORM
+ Have vague "feelings" if he hear lies.
+ Feel urges to punish liars (often in the form of feeling irritated/annoyed/headaches, depending on severity), but they're easier to resist in human form.
+ Under a powers restriction spell, which make it easier to control his urges to kill.
+ He still want to kill criminals, particularly murderers