Well, I that's just what I learned from Japanese Crossover Fanartists at Pixiv.
The one with the most "Similarities" is the REAL "Couple".
Just like how Order-Sol being coupled with Fiona, even though their Personalities are different. Sol is Cold-Hearted, and Fiona is really really Kind-Hearted. Their Similarity is their Big Sword and Fire Attacks.
And how Ky is coupled with Saki, even though their Personalities are also different. Ky is really really Kind-Hearted, and Saki is somewhat Snubbish. Their Similarity is being BOTH Lightning Users.
Jin and Weiss have a LOT of Similarities compared to others. Jin's attitude is Identical to Weiss before he goes all Psycho because of Ragna. Both are Duty-Bound, and has always been Loyal to their own respective Organizations. And being the 2nd Protragonists(Ragna and Heart being the 1st). So yeah! You get what I mean.
Just like Many Touhou Characters being Paired up with other Characters from other Series(Guilty Gear, BlazBlue, King Of Fighters, Last Blade, etc.)
Eh? But Jin and Weiss also have the same Positions, BOTH are what I call "Badass Military" = (Military Disciplined).
Hmm... Nope, I don't think Nightmare Fiction/Endless Despair is Identical to the Weiss VS Scharlachrot. You're pairing Scharlachrot with Hazama right? The problem is, Ragna is the one who is after Hazama/Terumi and wants to KILL him, not Hazama who is after Ragna. Which is opposite of Weiss and Scharlachrot, in which Scharlachrot is the one who keeps getting in the way of Weiss. Although, I admit that Hazama and Scharlachrot are BOTH Antagonists to the other Characters.
By the way, you paired Carl with Dorothy because of their Clothings right? Their Clothes are close to being Identical, and they BOTH wear Top Hats!
Edit: I forgot to mention 1 more Similarity between Ragna and Lieselotte: BOTH have White Hair!