The huge presence of Harley Quinn in DC Comics' latest film, "Suicide Squad", may have put an end to a widely believed major theory that fans of DC held with regards to the fate of Robin and the true identity of The Joker.
During the film, there is a scene in which Harley Quinn's history and specialty have been outlined in the form of a flashback, where she is seen to be majorly involved in murdering Batman's sidekick, Robin. However, a fan theory popped up before the screening of "Suicide Squad" according to Tech Insider.
The fan theory, which has become increasingly popular, suggested that Jared Leto's version of the Joker in the extended universe of DC is the same person as Robin. Leto's own Joker might actually be the second Joker to spring out in the whole storyline.
There are three aspects and factors that determine the possibility of this assumption. For one, this version of the Joker has a notably different appearance compared to the previous Jokers, mainly because it has a heavily tattooed body. The second factor is that Leto's Joker seems to be in the same age as Robin, seeing as Ben Affleck's Batman is on his 40's, while Joker looks like he is on his late 20's.
The third theory rests in the second latest DC film to be released, "Batman vs. Superman: Dawn of Justice". During this particular film, a battered suit of Robin can be seen, with the spray-painted phrase "Hahah joke is on you, Batman". This is possibly borrowed from the animated film "Batman Beyond: Return of the Joker".
In that film, Robin is kidnapped by the Joker and is psychologically tortured into insanity, where Robin is postulated to become a deranged "Joker Junior". The real Joker then dies, but Robin is forever damaged psychologically and never recovers in the process. This deranged Robin might just be who Leto is playing.
This theory falls apart, however, when Harley Quinn's bio is taken into consideration, where she assisted in murdering Robin, and that Leto's Joker is also involved with it. This would mean that Robin is truly dead in the extended universe of DC, and that the role of The Joker will not be wasted in future Batman films, according to Cinema Blend.
Here is a video explaining the Jason Todd / Joker fan theory.