Amnesia is a recurring plot device in the Final Fantasy series. The type of amnesia usually depicted is retrograde amnesia, where the afflicted person loses their past memories but is able to function normally and retain new memories. Other than this however, the cause of the memory loss varies but universally is used to conceal some secret about a character's past integral to the plot. How they recover their memories also varies.
In the Nintendo DS release, Luneth, Arc, Refia and Ingus have forgotten they were born on the surface world and were brought to the floating continent and given to foster parents by Cid Haze. Desch lost his memories while being in suspended animation for a 1000 years. He doesn't regain his memory until the party reaches the Tower of Owen.
While sailing to Baron, Leviathan attacks the party's ship and Yang is washed overboard, awakening on the Baron shorelines without his memories and being appointed a captain of the guard due to his Monk training. When Cecil arrives in town Yang and his men attack him, and Cecil and his allies defeat Yang in battle, the blows restoring his memory.
Galuf suffers a blow to the head prior to meeting the playable party and loses all his memories save for his name. He recovers a bit of his memories over the course of the game but does not remember in full until his granddaughter Krile appears, the sight of her restoring his memory. Due to his injury Galuf feigns head trauma and swoons as a running gag when he acts questionably.
Terra begins the game with no memories save for her name and a few brief recollections of the Gestahlian Empire, due to the Slave Crown used to control her mind distorting her thoughts. Once the magicite Maduin is brought to her, Terra's mind clears and she recovers all her memories.
Gau's Father has repressed the memories of Gau's birth, convincing himself he never had a wife or son in real life, and believes that in a dream he was married and lost his wife giving birth to a "demon child" he abandoned on the Veldt.
Cloud suffers from a complex series of memory disorders for much of the game. After impressing Zack and Tifa's memories of him combined with his own ideal image of himself as a SOLDIER, much of Cloud's memories distort into a false reality where he joined SOLDIER and became Sephiroth's partner, forgetting Zack even existed. However, Cloud's memory is still full of inconsistencies and holes that he is unable to explain. It is only after Tifa journeys into Cloud's Subconscious that the two of them are able to reconstruct Cloud's true memories and restore his mind.
In this game it is explained that prolonged usage of Guardian Forces results in memory loss due to the GF's presence in the controller's brain residing in the same areas that govern long-term memory. Their loss of memory isn't revealed until Irvine reveals to Squall, Quistis, Zell and Selphie that the five of them along with Seifer were orphaned and raised by Edea Kramer at a seaside orphanage. As the five grew older and were sent to Gardens to train as SeeDs, they began to use GFs and forgot about their childhoods, Irvine remembering due to not using GFs until he meets up with the party.
Both Zidane and Garnet have no memories of their early childhoods. Zidane was abandoned on Gaia by Kuja at an early stage of life and remembers nothing of Terra save for a blue light. Garnet meanwhile was born in Madain Sari as "Sarah", and she and her mother fled to the Mist Continent by boat when the village was destroyed. Garnet's mother perished before arrival, and the King of Alexandria found Garnet and took her as his own child to replace his recently deceased biological daughter, also named Garnet.
Sir Fratley was Freya Crescent's lover in Burmecia who left to travel the world and never returned. Freya sets out to find her lost love, but when she does, she finds out Fratley is suffering from amnesia and does not remember her. It is implied they become a couple again at the end of the game, although Fratley never gets his memory back.
Tidus fakes amnesia for much of the first part of the game, in order to make up for his lack of knowledge of Spiran culture.
During the War of Transgression, 700 years prior to the game's start, Fang and Vanille were tasked to become Ragnarok and lay waste to Cocoon. Fang ended up becoming Ragnarok alone and her incomplete Ragnarok cracked Cocoon's shell, at which point both Fang and Vanille were placed in crystal stasis by Goddess Etro.
700 years later, Fang and Vanille recover from their crystal stasis, but Fang has lost all memories of her previous Focus and Ragnarok. Vanille, wanting to protect Fang, hides the truth from her. Although Fang never truly recovers her lost memories, she is able to piece together the true events from the suspicious way Vanille is acting.
As a time traveler from 700 years in the future, Noel Kreiss possesses knowledge of the events between Serah Farron's time and his. However, his memory is affected by the paradoxes and corruption of the timeline, and he is unable to recall some basic historical facts which would help direct him and Serah on their journey. The effect gets worse as the story progresses.
Mog, Serah and Noel's companion on their quest, also lost most of his memories when he fell into a paradox in the Ocean of Time. By solving the anomalies in Oerba the player can help Mog remember his past.
In this game, warriors summoned to World B to fight arrive with no memories. As they fight their memories of their past lives and worlds return to them. However, if they are defeated in battle, Shinryu absorbs their memories of the cycle they were fighting in and revives them in the next cycle. The warrior retains the memories of home they recovered but has no recollection of the previous cycle. Those who are not defeated do not have this occur and they retain their memories into the next cycle. However, once defeated all memories of previous cycles are absorbed.