Toxic Morty is the embodiment of all qualities Morty deems negative.
Toxic Morty was fearful and self-depreciating, as well as entirely submissive to Toxic Rick. Created as the result of Rick and Morty having all their cognitive toxins removed in an intergalactic day spa. He helped Toxic Rick rebuilt the capsule to take their places as the real Rick and Morty. While on their mission to toxify the whole Earth, he was shot three times by Detox Rick in each knee and right arm as a torturing mechanism to make Toxic Rick and Morty reconnect with them. Three weeks later, Rick injected Toxic Morty's essence into Detox Morty, thereby reverting Morty to his normal personality.
He and Toxic Rick appeared in "Rest and Ricklaxation".
While it was never explicitly shown (likely due to him playing only a sidekick's role in Toxic Rick's plans) Toxic Morty is implied to have been very conscientious and moral, since these qualities were explicitly absent in Detox Morty, who at one point ordered Detox Rick to kill Toxic Rick. This means that Morty views his conscience as a negative or perhaps limiting quality.