AruAni

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

Canon
In the "sudden visitors" OVA Annie and Armin are together for the whole of the episode while trying to assist Jean.

While having their gear inspected, Annie subtly tries to persuade Armin not to join the Survey Corps and recognises his strong spirit, showing some level of care as well as respect.

On the Survey Corps' 57th expedition Annie spares Armin's life twice whilst in Titan form even though she slays everyone else. This is despite the fact that Annie was probably aware that Armin would be able to use his wits to identify her. Armin, however, waits two months before saying anything because he desperately wants to be wrong for once.

Armin emotionally blackmails Annie into helping them even though she knows it is a trap. Annie admits that she couldn't understand as to why she didn't kill Armin before, and she seems hurt by his betrayal. During her fight with Eren, Annie spares Armin's life yet again despite Armin trying to goad her into killing him.

As an outcome of the Battle of Shiganshina Armin inherits Bertolt's memories and has since gained a much deeper understanding of Annie and her past. Four years on Armin is seen confiding in Annie, even though she is still in her crystal form, and makes an emotional appeal for her to respond. He doesn't seem to blame her for the things she has done having now experienced the anguish of being the bad guy.

Fanon
Some theorise that, given that Bertolt loved Annie, it is possible that Armin now has romantic feelings for her. In addition he is probably the only one who still visits her.

Fandom
AO3