This seems to be the case. Took me a while to figure out, my free UR was Tohsho Daigongen and I couldn't tell if Endurance was ever doing anything. Spent a lot of time looking around in the game and couldn't find anything, even googled "4 demons of tokugawa" and found nothing. Then while building affection with Honda Tadakatsu I noticed endurance triggering at the beginning of every battle. Honda's profile lists out the four heavenly kings. I did have Il Naomasa, I didn't notice it triggering endurance, maybe I wasn't paying attention, I sacrificed her without realizing it. I haven't seen the other two.
I'm starting to feel like rarity doesn't really matter much. There seems to be very little stat difference, just a few point in Power, Leadership and Intelligence, and you can buff all those quite a bit using research and items. The skills seem to be the only thing that matters, and many of the SSR and URs have powerful skills that require SRs and Rs to trigger. Like out my five different SSRs 3 of them need a rare to fully function. Toyotomi Hideyoshi requires Nene, Tokugawa Ieyasu requires the 4 "demons," and Hohjo Ujiyasu along with several SRs are all built around a R, Hohho Suhun. The Hohjo/Hohho units are pretty interesting, it's like a SSR and 2 SRs that all focus on boosting the reflect of a single R unit, and it looks like it can get close to if not over 100% damage reflected on her.
When people attack you they do attack your defense team. Or at least they should, they do for me. I'm pretty sure your defense leader is also your account avatar for your friend list and whatnot.