I never realised why I kept confusing “Suicide Squad” with “Birds of Prey“. The girl in both movies is actually the same comic character: Harley Quinn.
The first “Suicide Squad” was made 4 years before “Birds of Prey” and indeed, in “Suicide Squad” we learn how Quin became the girlfriend of The Joker.
Gotham is plagued by “metamen”, people with extra abilities. Superman stopped doing his job, so a government agency decides to put the worst “metahuman” villains, that are all behind bars, in a team to fight current and future “metamen” mayhem. So we have a group of anti-heroes and the main characters of “Suicide Squad”.
The result is an alright thin story action comedy with alright jokes and descent action scenes introducing enough new characters for another few spin offs.