Killer Sans, more commonly referred to as Killer is a Sans from the Something Different AU. He also is one of the Bad Sanses, led by Nightmare. 

Killer is a rather emotionless person, but guilt-ridden by his actions in his timeline, by killing all of his friends. But most know him for his overtly romantic personality, as he known to flirt with any and sometimes all girls. He’s also known to make other extremely uncomfortable because of these actions. He's also batshit insane. 