Somebody who uses you for their personal gain and doesn't care how it affects you. They see that you are a kind person and they use you for that. Mr. Toxic is hella toxic so if you know one, stay away!

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.