Killer Sans

Dull/Darkened blue jacket with a fluffy hood (often depicted as having a black stain on the chest), black turtleneck, shorts with inverted colors, shoes instead of slippers (sometimes with no laces or untied ones)

Lazy eye socket, usually his right one is "droopy" and half closed, black liquid dripping out (depending on stage), red soul

The black stuff often seen dripping out of Killer's eye sockets is corrupted magic, some people interpret it as pure HATE coming from Chara but this is not the case, the tears go away when Killer's soul is regulated (upside down red target heart), possibly because his soul regulates the magic better, like how his emotions are regulated better in that state

He is very thoughtful but not to the point of feelings, he hates being belittled or compared especially to other Sanses. Generally a negative to neutral character, although Killer loves cats and had two before Nightmare killed/attacked one for making Killer positive. Killer also seems to suffer from some type of personality/identity disorder similar to DID. He'll have little breakdowns and freak outs when he's beginning to feel again and is reminded of the pain he's caused. Not only that but he also has a shadow-like Papyrus that will sometimes appear to him, possibly symbolizing his guilt going into overdrive as his sins crawl up his spine. He's hard to read and often unpredictable, his emotions are often suppressed or out of control, when they are regulated however he has a difficult time with them, often feeling numb or upset

Sans is surfaced, in this stage others can normally talk to him, he is generally approachable

Classic Killer, should not be trusted, emotions are suppressed and he's unpredictable

"Crazy Mode", he will hurt anything and anybody in his way whether he means to or not, very violent

Details about this stage are vague but it seems it is most dangerous stage, and it is shown that it can get worse, a theory suggests it's when Chara wins, in oppose to stage one where "Sans" wins