Shingeki no Kyojin: Kuinaki Sentaku

Midnight sun episode scene connection  

Similar dynamics and themes as well as switched character designs. The save me/catch me if you can/ I gave you my word theme 

Similar dynamics and themes as well as switched character designs. The save me/catch me if you can/ I gave you my word theme 

Similar character dynamic 

Similar character dynamic  

Similar character dynamic  

Similar character dynamic 

Terra Formars Bugs 2 and Attack on Titan: No Regrets are both prequel OVAs about the backstory of a main character. Both OVAs show the tragic past of the character. The threat in both series are a human like species who cannot communicate with humans. This species also wants to understand human technology, while being physically stronger than humans. 

"Gargantia on the Verdurous Planet Specials" / "Suisei no Gargantia Specials" and "Attack on Titan: No Regrets" / "Shingeki no Kyojin: Kuinaki Sentaku" are both prequels to action shounen series that expand more on the backstory of side characters. For both of these series tie-ins, additional context is provided and mature issues are tackled. 

The story is focusing on "Levi" humanity's strongest and "Accelerator" Academy City's strongest. 

Similar character dynamic  

-both about the people who live in the underground or the inner city and the walls beyond that - 2 characters with a close relationship who save each other beyond the walls - both are called “inseparable duo” “Desolate wasteland beyond the walls of the supposed utopia…Shion decided to take him into the night and protect him” Now why does that sound familiar  

Both are short-lived prequels (ovas) of two popular animes. Both stories tell the tragic past (full of violence and blood) of two characters who are almost invincible sword fighters.