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 were together for the whole of the episode while trying to assist Jean.

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

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 Armin would be able to use his wits to identify her. Armin, however, waited two months before saying anything because he desperately wanted to be wrong for once.

Armin emotionally blackmailed Annie into helping them even though she knew it was a trap. Annie admitted that she couldn't understand as to why she didn't kill Armin before, and she seemed hurt by his betrayal. During her fight with Eren, Annie spared Armin's life yet again despite Armin trying to goad her into killing him.

As a result of the Battle of Shiganshina Armin inherited 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