Right at the show of Pilgrim's band, the "Sex Bob-omb", during a "battle of the bands", he is attacked by the first ex, Matthew Patel, and after defeating him, Ramona tells him the conditions he needs to fulfill in order to be her boyfriend.
The story goes as Scott defeats the Evil Exes one by one, learning more about their previous relationships with Ramona. It gets worse, however, when he finally reaches the final ex, Gideon Graves, who is also the manager of his band.
They meet at the Chaos Theater raised by Gideon and both fight over her. However, even confessing his love isn't enough as Pilgrim is defeated and sent into a limbo. But he manages to escape due to an extra life (the movie has many game references and mechanics) and finally defeats Gideon, starting his relationship anew.
The league was founded two weeks prior to the events of Volume 1 after Gideon found them online and manipulated them into joining him by making them think Ramona was the cause of their unhappiness.
Matthew Patel - The first ex, Ramona's shortest relationship. He is an avid Indian fighter with the ability to levitate, summon fireballs and a demon known as a "Demon Hipster Chick".
Lucas Lee - The second ex, a professional skateboarder and also a professional movie star. A very selfish man, his arrogance can sometimes lead to very dangerous stunts, and much to Scott's delight, his downfall.
Todd Ingram - The third ex, a bass player for a band named "Clash at Demonhead". He follows a rigid vegan diet that somehow resulted in his psychic powers, and as a bonus, dates Scott's ex-girlfriend, Natalie "Envy" Adams. But at the same time he eats non-vegan foods in secrecy, he cheats on Envy with other girls over and over. The only way to beat him is to take out his powers.
Roxanne Richter - The fourth ex, and the only woman/lesbian in the group. She is a "half-ninja" with a severe inferiority complex, who taught Ramona much of her subspace powers. She is also the only ex Scott refuses to fight, due to him being unable to hit girls or anyone with a sword.
The Katayanagi Twins - The fifth and sixth exes, a pair of Japanese twins named Kyle and Ken who constantly finish each other's sentences. They are both famous Japanese musicians and roboticists who use robots to fight Scott, also using their music as an area-wide attack. They left Ramona after figuring out she was cheating on them with each other.
Gideon Gordon Graves - The seventh and final ex, the League's leader and founder, hinted to be Ramona's most recent boyfriend previous to Scott, and the main antagonist in the series. He is the owner of the Chaos Theatre, never seen until the end of Volume 3 in the books. Gideon sometimes appears to be a scientist, and also works as the Sex Bob-omb's manager under the alias of "G-Man". His power is called "emotional warfare", an ability that traps people inside their own minds with their own issues. He used this power to constantly be with Ramona inside her head, causing her to never truly leave him. In the movie, he also keeps Ramona close to him, only this time he uses microchips implanted on her head.
Despite being evil, the League does seem to abide by certain unwritten rules, namely that they can't gang up on Scott all at once. Instead, each member must challenge Scott in the order they dated Ramona, however, Kyle and Ken Katayanagi violate this rule by fighting Scott side-by-side (though they justify this by pointing out that Ramona cheated on them at the same time). Roxie also violates this rule in the film when she tries to attack Scott before his encounter with Todd Ingram. The League (with the exception of Gideon) also tries not to inflict any physical harm on Ramona herself - as she is the reason they exist in the first place.
Whenever one of the members of the league is defeated, a prize is earned in coins. This is a reference to River City Ransom and No More Heroes, where coins are earned every time an enemy is defeated.
Right before every fight in the movie, a versus symbol (VS) appears between Scott and the evil ex.
In the trailers for the movie, when the faces of all the Evil Exes are shown, Lucas Lee's face is at the center space instead of Gideon.
Though they all appear to die and disintegrate into coins after they are defeated, they don't at all die. They just simply respawn back to their homes, having learned their lesson. They also wouldn't consider a vendetta on Scott anymore since they would find it boring and not worth the effort anymore (and traveling from the US to Canada would cost a lot. They all live in the US.) Details here and here .