AruAni

AruAni is the het ship between Armin Arlert and Annie Leonhardt from the Attack on Titan fandom.

Canon
Annie spared Armin's life twice whilst in Titan form, even though she slayed everyone else. Annie was probably aware that he would be able to identify her since he's very smart.

Armin emotionally blackmailed Annie into working with them even though she knew it was a trap, thus implying they got close enough for him to know how to get under her skin. More than Eren, who is noted to have been getting combat lessons from Annie.

Armin waited two months before saying anything about the female titan's identity because he wanted to be wrong for once.

Once again, during the rematch of Eren vs Annie, Annie spared Armin's life.

Armin confided in Annie, even though she was still in her crystal form, and begged her to say something in response. This scene showed that Armin does not blame the warriors for their past actions. Through his own similar experiences, Armin has come to realise that Reiner, Bertolt and Annie had no choice.

Fandom
AO3