Attack on Titan has never been shy of taking on socio-political issues and bending them to the shape of the manga/anime's horrific dystopian story. At the same time, the nature of Attack on Titan's premise and story doesn't leave a lot of room for romance - at least not outright.
That's not to say there haven't been hints of serious romantic subtext in Attack on Titan; in fact, the latest episode of Attack on Titan season 3 may have just confirmed one of the most obvious (but never confirmed) LGBTQ relationships on the show: a romantic connection between Queen Historia and Ymir!
Attack on Titan episode 58 is called "Attack Titan" and it reveals the important and game-changing history of Eren Yeager's father, Grisha, and his mission to infiltrate the Walls of Paradis Island and steal the crucial Coordinate Titan, which controls all other Titans. Part of that secret history was shared with Eren and the Survey Corps through Grisha Yeager's journals, including the history and true power of the Coordinate Titan. However, another piece of Titan history is shared with Queen Historia, through a final letter written to her by her one-time protector, Ymir. That letter reads as follows:
"To my dear Historia: As I'm writing this, Reiner stands next to me. It's pretty obvious that he's sneaking peeks at this love letter. He can be such a creep. No wonder he's single. However, he did promise me he'd deliver this letter to you. He says it's payback for saving them that one time. I'm sorry about then. I never imagined I would end up choosing them over you. It won't be long till I die. But I have no regrets. That's what I'd like to say, but there is one thing: It's that I wasn't able to marry you. Sincerely, Ymir."
Historia and Ymir's relationship was always pretty ambiguous - especially in their shared arc during Attack on Titan season 2. It was always clear that Ymir had a serious infatuation with Historia (aka Krista), as she risked herself time and again to protect Historia, save her, or outright tried to run away with her. It was never really shown that Historia reciprocated those feelings, thought - at least until this scene in "Attack Titan".
Hearing Ymir's one regret in the final "love letter" visibly fills Historia with emotion, even though her status as Queen doesn't let her breakdown and show it in front of the Survey Corps. Historia does get one last intimate moment with Ymir, as touching the letter allows the royal family member the privilege of seeing to see a vision of Ymir's life - including her apparent last moment, being chained up as what is seemingly an offering for a pure Titan, with a mysterious figure standing over her.
Now Historia may have all the motivation she needs to avenge her fallen (friend?)(true love?). And while Attack on Titan may have never made this an official LGBTQ relationship, it's pretty safe to infer just that.
Attack on Titan season 3 part 2 is now streaming weekly on Hulu, Funimation and Crunchy Roll.