Soukyuu no Fafner: Dead Aggressor

Both Evangelion and Fafner employ mechas and monstrous creatures to develop the characters of the pilots and their inner conflicts. In addition, both series have a penchant for the philosophical: Eva tends to be darker and perhaps more confusing while Fafner is more straightforward and optimistic.  

The character design was done by the same person. Although both are Mecha-animes I wouldn't say that the plot is all that similar though. 

Both are mecha shows set in a post-apocalyptic backdrop with large, alien enemies and similar themes. 

Both deals with alien invasion and (near) destruction of mankind. Both are equally depressing but have a very solid plot and character developments. Soundtracks are great too. 

Humanity's existence put at risk. Both series features the theme of survival as humanity faces the threat of alien beings. There is a decent amount of action as humanity decides to fight back to survive. The main male protagonist becomes a prominent figure as they spearhead against the mysterious invaders. And speaking of mysterious, there is a lot of questions of thought regarding origins, the mecha, and even characters themselves. If you are seeking a sci-fi series with warfare themes taking place in the future, then be sure to give these two shows a try. 

Similarity : Both anime featuring a unique system which enables two or more users to share experiences, abilities, and skills; all while increasing the abilities and output of theirmake their mecha units as well. Siegfried system (Soukyuu no Fafner) : Operated by Minashiro Soushi This system taken to the extreme, not only do the pilots synchronize with their Fafner units, but they are also linked with Soushi mind via the Siegfried system, which allows him to guide them while forcing him to feel all the pain they feel. Coupling system (Buddy Complex) : This system allow two Valiancer pilot connected their mind which can make unlock special power such combine their aeroscales, back-to-back, and manifest a small force field that could dissipate oncoming attacks.  read more 

The focus on the use of children as soldiers to fight while the adults are forced to just watch on the sidelines are true for both these series. In Majestic Prince, we have the genetically engineered children who were raised for the sole purpose of being exceptional soldiers in the fight against the Wulgaru. In Fafner, the children on Tatsumiyajima were all genetically engineered to be able to pilot the Fafner mechas. 

Alien invasion. Alien assimilating with mankind. Communication. Fafner much better in terms of story and soundtracks. 00 better in terms of actions and animation.  

Fafner and Bokurano deal with young pilots who are forced to ride mechas in order to defend their homes at all costs. Fighting an enemy whose true nature proves to be ambiguous, the children in both series are manipulated by corrupt adults and must find the strength to deal with the chaos and violence that threatens them. Bokurano is perhaps a greater tour de force than Fafner (with its relatively tame content), but in both cases the personal relationships established between the pilots, each other, and their loved ones are at the forefront of the narrative, providing a solid emotional background for the action. Both series highlight the role of family as they develop character-driven stories that are bitter sweet and very moving in their emotional rapport.  read more 

Both are stories in which a teenager unexpectedly becomes a pilot of a mecha and has to fight to save the Earth, while also retaining some level of an everyday life. They also both have hints of a romantic subplot, which become more important as the series go on. Solving conflicts through communication is a common theme as well. An important thing to note is that Fafner is more action focused, while Xenoglossia is mostly focused on the everyday lives of the pilots, however there is a good amount of character focused drama in both. Heavy inspiration from Evangelion in both shows as well. 

Both mechas based around high schoolers. Valvrave is clearly more aimed at comedy though but still has lots of twists. The two main girls come across as very similar too. 

I think D.Franxx draw alot of inspiration from classics like Rahxephon, Evangelion and Soukyuu no Fafner. But in terms of plot wise it is almost 80% similar to Fafner. All deal with frustration of teens growing up and piloting mech instructed by adults to protect island, I mean Earth. 

Combining elements of advanced technology (mecha) and sci-fi themes, both of these series involving an invasion to a planet where the main characters fights back as resistance. The main male protagonist becomes a driving force as he takes charge on dealing with the invaders alongside others. There is a state of warfare as humanity/earthlings fight for survival. At the same time, there are also mysteries regarding the origins of the mecha and characters. Recommended for anyone into sci-fi/mecha with warfare themes. 

Both are mecha anime series, where the human race must fight for their survival. And there are major character development between friends. 

In both anime human almost face extinction because their cant birth baby again but they are trying to solve that problem with any cost.  

Alien invaders filled the earth. Both series contain comedy, action, drama, some tragedy, and later romance. Both series also contain a cast of characters involved with the military.  

Both are set in a world where monster-like aliens have invaded the Earth and bringing humanity to the brink of extinction. Humanity fights them back in giant mechas. Also, the human world is not entirely united as one to fight off the aliens together. 

Special persons controlling special humanoid looking devices (i.e Robots ^_^ ) for protecting something important. Similar plot and atmosphere, altho Fafner has a much darker atmosphere than Candidate for Goddess  

They are both mecha animes and the people are choosen to piolet them (In different ways). And the city in Kurogane is just like the island (During episode 18). Though Kurogane no Linebarrels isn't as serious as Sokuyuu no Fafner but its a mix :) 

has the same theme as eureka seven.... you know, the end of days. 

Both anime contain mecha which have bad side effects on the pilot such as making their life shorter and their mental health unstable. There are also characters which have similar kind of mysterious aura and some part of the story are also similar. 

Fafner and Evangelion are kindred spirits, both explore the human elements and character interactions that are forged in extreme situations, employing the mecha genre as a means of developing the young pilots and the ordeals they go through. You are (not) Alone, in its skilful retelling of the first six episodes of Evangelion, is closely tied to the initial phases of Fafner.  

Eventho on the surface these two titles have nothing in common: one is mecha [Fafner] and the other is dystopian fantasy [No. 6], the reason why I'm linking the two is because of the very gay [but not really gay] subtext of these titles. These titles offer male protagonists whose relationships [with each other] hover in the grey zone that is beyond a fujoshi's vivid imagination "slash" but not quite officially shounen-ai. Though, not officially declared shounen-ai by the creators, there's also no statement denying the characters are not gay, leaving their sexuality or relationship with each other.....open to speculation [which is usually perceived as gay]. read more 

Both by the same studio and feature an opening theme by Angela. Both are shows featuring a combination of mechas and creatures. Both are on the losing end of a war with futuristic technology. Fafner is the more serious of the two with a much sadder storyline although you can emphathise with Age more in Heroic Age.  

Similar dark theme + similar crystals behavior  

The Evangelion universe and Fafner are intimately linked; both tackle the mecha genre from a philosophical and highly emotional angle. Fafner's resolution should be seen side to side with the summit of NGE that EoE represents, both are powerful in their own ways: Fafner offering a more optimistic perspective to EoE's ambiguity. 

Sci-fi/drama with a mecha background and a ton of plot twists. 

I don't know why but while I was watching Terra e... I remembered to Soukyuu no Fafner If you like one of them then I think you will like the other too. ^^ 

The character design are the same, both of them are made by Hisashi Hirai. And they are both mecha anime.