Notes from author
Part of a series I'm going to do, covering all levels for co-op summoning, conservatively. The necessary levels are 12, 36, 65, 101, 145, 198, 263, 343, 441, 561, and 707. If you have a character at each and every of these levels, then you will be able to be summoned to anybody, at any level, with one of these.
This build itself doesn't matter as much. I'm going to base it around Giant Dad, but, obviously, you can use any build you want that fits the soul level.
Now for build-specific details.
Here we have much more freedom, and can meet the stat requirements for most weapons and spells, assuming you prioritize them. Finally, we can get 40 endurance, maxing out our stamina. With 40 endurance, we still can't light roll with full Giant Dad, but we can replace Havel's ring. If we put the points into other stats, however, we can begin to benefit off of scaling. We cannot yet use soothing sunlight, but we can use bountiful sunlight, with 36 points in faith. Hopefully, the host should be more competent at this point, but his survival should still be top priority.