AruAni

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

Canon
On the Survey Corps' 57th expedition Annie spared Armin's life twice whilst in Titan form even though she slayed everyone else. This was despite the fact that Annie was probably aware that he would be able to use his wits to identify her.

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.

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

During the rematch between Eren and Annie, she spared Armin's life once again.

Armin inherited Bertolt's memories and has since gained a much deeper understanding of Annie and her past.

Four years later Armin is seen confiding in Annie, even though she is still in her crystal form, and pleads with her to say something in response.

Fanon
Some theorise that, given that Bertolt loved Annie, it is possible that Armin now has romantic feelings for her.

Fandom
AO3