The Joker
Basically,He's a down on his luck loser Factory worker who's trying to get into comedy,but can never find any work,do to his lack of good material.Loosing all hope left,He gives up any hope of entering standup,and returns to his job.
One night,while He's working,some escaped inmates from the Narrow incident turn up at the factory,hiding from the police.Batman,who shows up to catch the inmates,mistakes him for one,and chases after him.The man,being terrified of Batman,gets so desperate to escape that he dives into one of the chemical vats in the factory,and seemingly dies.
As Batman and the police round up the escaped inmates,the loser crawls out of the vat,and decides to go home,after all of the stress he went through that night,not realising what the chemical did to him.
Back at his obviously crummy apartment,He comes in,and walks into the bathroom.As he goes to use it,He catches a look at himself in the mirror,only to see that his hair is bleached a dark green,and his skin has been turned snow white.
Distraught by the image of himself as a clown,the man finally goes off the deep end,going into a rage of laughter,while breaking the mirror apart with his fist.Loosing even more of his mind,the man,inspired by the image on a Joker card,sitting on the sink,takes one of the glass pieces,and basically mutilates his mouth until it resembles a bloody,almost demonic looking permanent grin.
Deciding,once and for all,that his life as a normal person is over,He vows to make Batman pay for forcing him into the chemical,by killing Gotham City as a whole,as Batman obviously wants to save it.
Taking another one of the glass pieces,He runs down to a local store(It'd be like a Goodwill store),kills the cashier,and picks out a purple suit,gloves,new shoes,some spats,and even finds an antique cane.He even takes a tube of lipstick,to cover up the deformaties of his mutilated grin.Realising he needs a new name to go with his new life,He eventually turns to the Joker card,once again,and comes up with the name 'Joker'.
Now,He has good comedy material.It's just unfortunate that it happens to be sick and twisted,and usually involves death.