During their time in training, Annie and Eren developed a mutual respect and interest for each other. She was his mentor and she explained him the contradictions of the military training system inside the walls, where the people with the best ability to oppose the Titans get the priviledge of staying far away from them. She trains him in her unique fighting style, and he is one of the few people to have managed to make her genuinely smile. When her mission required it, she ruthlessly pursued him and fought him twice. Her betrayal deeply shakes Eren.
Erens perspective
Eren greatly admired Annie, and was trained in her unique fighting style during their years as trainees. He considered her to be a kind person beneath her cold exterior and credited her with making him aware of the contradictions within the military system. However, he was eventually horrified to learn of her identity as the Female Titan. Even as Annie herself transformed in front of him, a part of Eren refused to accept her betrayal; and he became so conflicted that he was initially unable to summon the will to fight her (Mikasa even went so far as to suggest that his hesitation was due to him harboring "special feelings" for her). Even as he went toe-to-toe with Annie, he continued to brood over what had driven her to slaughter their comrades and displayed far less anger and contempt towards her than he did for Bertolt or Reiner.
WARNING!!!!!THE JAPANESE AOT STUFF THOUGHT THAT EREN HAS ROMANTIC FEELINGS FOR ANNIE EVEN THE ENGLISCH STUFF AND THEY WERE GOING TO PUBLISH THIS BUT BEFORE THAT THEY WANTED TO ASK HAJIME ISAYAMA FIRST ABOUT THAT IN ORDER TO BE CONFIRMED BY THE AUTHOR HIMSELF BUT WHEN THEY ASKED HIM ,HE QUICKLY DECLINED OR REFUSED ABOUT EREN FEELINGS FOR HER AS ISAYAMA STATED THAT THEY WERE ONLY FRIENDS AND HAD RESPECT FOR WACH OTHER NOTHING MORE NOTHING LESS .
Okay the first evidence to be found is that when Eren couldn't transform into titan Form after the transformation of Annie.Yes this might be a nice okay evidence but it's not because he had romantic feelings for her ,it's because he couldn't believe his friend and his mentor or trainer betrayed him like this.2)He wasn't so angry with her like Reiner and Bertholt. Yes this is true but she wasn't the cause of his mother death like Reiner and Bertholt remember. 3)He didn't eat her in episode 24 or 25 when he was in berserk mode .Yes this is true but it was because he couldn't believe the cold-hearted Annie was crying . (YES BUT MIKASA NEVER CRIED AND ALSO HE DIDNT RESIST FROM ATTACKING MIKASA).First of all ,Mikasa did cry three times actually or more actually first when she learned that Eren was dead ,second when she saw in her own eyes Eren wasn't dead when he came out from the titans nape and third when she learned that Eren was kidnapped and many other times that I will say later )(And Eren resisted his titan Form from attacking Mikasa as it could be seen when he his titans hand was shaking very much )but I have another theory:
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This theory is more strong for me than any other one this might be the reason why Annie teached one of her techniques to Eren.
