In dot arena, lumos would prioritize linking with heroes that had lower strength than lumos (if all above its completely random)
So for example, tholin, riley, lumos, dokras, and gale. Lumos would always link with dokras.
However, this isn't dot arena and we don't use strength. Strength in that game was attacked power and health. Maybe someone can do some testing to figure out if this holds true in soul hunters as well
It should be noted that this isn't dot arena and there is also the chance that lumos could always be 100% random