The Cleaner is a professional clean-up artist, similar to Winston Wolf in Pulp Fiction. While bearing an obvious resemblance to Mr. Clean, he sounds like Elvis, suggesting that as with the Guild that the OSI employs former rock stars including those presumed dead.
In addition to doing a "Martha Stewart" on the hotel room where Samson eviscerated Le Teur, leaving it spotless with just a hint of lemon (possibly his calling card), he also breaks Brock Samson and the Venture Family out of jail.
For weaponry, he has been shown brandishing two spray bottles full of a highly corrosive liquid that melts skin and bone on contact.
While his origin is shrouded in mystery, he has some connection to both Molotov Cocktease and Col. Hunter Gathers- he is last seen in Season 3 in a car with Cocktease, and in The Family That Slays Together, Stays Together (Part 2), Brock instructs Hank to find the number for The Cleaner in a directory of phone numbers, which Hunter describes as "a list of friendly contacts- all ex-OSI who have gone off the reservation, like you."