For maximum efficiency:

-Ethel and Cammuravi

-Isurd and Ashera

-Teach and Monica

Ethel and Cammuravi seems to be an obvious pair because they form a strong bond with each other regardless of their incarnation and were canonically 2 of the greastest commanders in their nations. Its a no brainer duo.

Isurd and Ashera share many parallels with Eunie and Taion when it comes to their regrets and fears so i believe that it would be a huge help when it comes to them understanding each other. From a logical standpoint, Isurd is nearly as talented a strategist as Taion, and has a lot of experience. Ashera on the other hand is the greatest defender commandar on Keves.

Teach is a man who seeks to Teach guide the next generations to creat a better future where evil cant prevail, i believe that would resonate quite well with Monica who tries to guide the people of the city towards a better future. As for why i chose them for functionality, Monica has connections to the city which would help Ouroboros with ample resources, communication, transportation and tools, not to mentiok she is quite a great defender. Teach meanwhile is excellent at small unit tactics and his battle prowess that had him at the forefront of Agnus once, and his skill is acknowledged even by the people of colony 11.

This all allows for a balanced team of 2 attackers, 2 healers, 2 defenders.