Throughout Captain Marvel, the titular superheroine is in constant battle with Talos - the leader of a race of shapeshifting aliens called Skrulls.
Over time, it is revealed that this entire conflict is one big, fat, red herring and the Skrulls are actually the good guys. During the climax, Captain Marvel discovers the aliens she was fighting with, the Kree, are her true enemies and their entire plan to enslave the Skrulls was orchestrated by her mentor, Yon-Rogg.
Although this is meant to be a big twist, it doesn't really work. In every other MCU film, the villain is established almost from the get-go. But in Captain Marvel, Yon-Rogg has ten minutes to go full-baddie.
Also, Yon-Rogg comes across as a complete wimp in the showdown since Captain Marvel defeats him with a single energy blast. There's no ten-minute battle. No moment where his victory seems imminent. She just knocks him out in one strike and calls it a day. Because he doesn't go full baddie until the third act and he goes out like a chump, Yon-Rogg may be the least memorable villain in the entire franchise.