During the first season of Attack on Titan, people were wondering who the Female Titan was. The female Titan was Annie Leonhart. She was an extremely powerful Titan who displayed mastery in hand-to-hand combat and possessed hardening capabilities, making it difficult for the Scout Regiment to cut her nape.
On one hand, they wanted to protect them from the titans that threatened their lives. On the other hand, they felt betrayed by humanity for how it treated them as outsiders and feared that if left unchecked, humans would eventually turn on them too.
After her identity was revealed, which resulted in her defeat by Eren Jaeger and Mikasa Ackermann, she crystallized herself into a coma to protect herself and the secrets that she held from being unraveled. Until her escape four years later, Annie was retained by the Survey Corps [Chapter 34, Volume 8].
Armin leads Eren, Mikasa and Annie to a tunnel, but Annie refuses to enter. Armin finally reveals his suspicion that Annie is the Female Titan, noting that she used Marco's omni-directional mobility gear to kill Sawney and Beane and that the Female Titan did not kill him during the 57th Expedition.
Annie was one of the first antagonists in Attack on Titan. While she originally seemed like an ordinary soldier from the 104th Cadet Corps that joined the Military Police, her comrades eventually learned that she was the inheritor of the Female Titan, one of Paradis' biggest threats.
Annie is genuinely selfish, due to having been abandoned at birth and being used as a tool by everyone she has ever known, even by her adoptive father who wanted a better life for himself through her. She has no interest in protecting strangers due to being unattached to most of humanity to an absurdly high degree.
Annie acquired her Titan powers while she was a Warrior candidate in Marley and was trained with using them by the military of Marley and by her father.
Armin's feeling for Annie are more subtle during Season 1 but it is possible that Armin has had feelings for Annie before her reveal as the Female Titan.
With the inclusion of understanding given to him through Bertholdt's memories and the emotions that coincide with those memories, Armin grew a greater sympathy for Annie's situation, and the continuous interactions he had with her built up his romantic feelings over time.
Bertolt Hoover
Bertolt has been in love with Annie since they were children, and both were Warriors from Marley alongside Reiner. It's unknown if Annie knows about Bertolt's feeling towards her.
Levi has the highest Titan kill count with a rumoured 200 kills, and at least forty on screen kills. For reference, the MC, Eren, only has twenty Titan kills on screen despite having the power to become the Attack Titan.
Four years later, there is still no sign of the Female Titan's return. Zeke speculates it was either stolen or Annie is still alive since there have been no children discovered on the mainland with the Female Titan's power. Somewhere underground in Paradis, Annie is shown to be still incapacitated within her crystal.
Founding Titan is the first and the strongest Titan of all the nine. If the Founding Titan is someone from Royal Bloodline, then it will possess limitless power. Eren Yeager was not someone of royal blood that's why he asked for Zeke's help to use the Founding Titan to its fullest.
In the Junior High anime it is heavily implied that Annie has a crush on Eren and the two of them bond over their shared love for cheese burger steak.
Trained from an early age by her father, she was sent to carry out a mission against humanity. As the Female Titan, she served as a major antagonist of the series prior to being exposed and defeated by the military.
Love Confession — Eren confesses to Armin that he always loved Mikasa, and can't bare seeing her love another man. Alternate Universe — Eren sends Mikasa a vision where Mikasa truly were to confess her feelings to Eren, the two then run away to live together as a couple for the remaining of Eren's final years.
8 Annie Was Chosen To Inherit Her Because Of Annie's Martial Arts Abilities. Due to Annie's experience in martial arts, Marley decided that she would be a good user of the Female Titan. At the same time, Marcel, Bertholdt, and Reiner were chosen to be the Jaw Titan, Colossal Titan, and Armored Titan, respectively.
Eren and Mikasa Finally Revealed Their Feelings
After years of unrequited love and hopeless pining, Attack on Titan's ending saw Eren and Mikasa confess their love in their own way.
In S1 there were hints that she did like him: Her letting him live is what got her caught. She wanted to be "a good person" for him and helped him try and smuggle them out.
Bertolt and Annie were both Warriors from Marley, and Bertolt developed romantic feelings for Annie. It is unknown whether Annie knows about this. The two, along with Marcel and Reiner, are sent to Paradis to retake the Founding Titan.
He screams and cries, begging his former allies to save him as they watch, with tears in their eyes. Bertholdt cries out for Annie and Reiner before he is consumed alive and killed by Armin. Armin has a nightmare about Bertholdt before he wakes up. He dreams that the Colossal Titan is crying.
This indicates that perhaps the point of this titan was not to be inherently female, but has abilities centered on crystallization. Therefore, if a man were to eat the human wielding this titan, he would possibly retain his masculine features, but his defining characteristic would be to crystallize.
The nine Titan powers were the Founding Titan, the Armored Titan, the Attack Titan, the Beast Titan, the Cart Titan, the Colossus Titan, the Female Titan, the Jaw Titan and the War Hammer Titan.
Levi managed to incapacitate the Female Titan, but at a crucial moment, the Female Titan summoned more Titans to come to her aid. As the horde of Titans joined, Levi impressively killed them instantly.