If Mitsuki is Oro's son (I made the link between them at the first draw of Mitsuli because of the physical similarities , so I'm not shocked , I mean , yellow eyes are pretty rare) , he may have Oro's natural affinities , he may be Yin/Yang.
When they make the teams , they like to mix abilities and affinities.
For exemple team 7 : Naruto is Yang , Sasuke is Yin , Sakura is Yin/Yang.
Another good exemple is the historical InoShikaCho , Ino is Yin/Yang , Skikamaru is Yin and Choji is Yang...
Sarada may be Yin , Boruto may be Yang , it may not be that easy to find a Yin/Yang kid of that age , Mitsuki came in the village , and here they found the perfect third team-mate.
Mitsuki will be the "Sai" of this team and will try to understand Boruto's emotional behaviour.