From my experience it doesn't make that much of a difference which character you choose if you play in Arrange since you will most likely have your spirit deployed the whole time and therefore have no difference in movement speed.
Gensuke's and Shishin's spirit have a more circular damage radius while Kosame's spirit deals damage to a more linear area, thus making her slightly weaker. I would say go with the character, whose hitbox you can measure best (for me that was Shishin).
Just my personal observations...
edit: the very last boss indeed has a pattern but it is a little bit vague. Observe the yellow balls closely and you will see that most of the time their trajectories stay roughly the same. What's random is their spin (clockwise/counter-clockwise), so basically you have to find a position where you can react to both cases. Keep at it!