Just remember episode 10 where we got a deep look into Homura we find out these close friends have been separating themselves entirely from everyone in order to fall curse to the contract dooming them to fall into a deep despair and become witches eventually themselves. And that this has happened since the beginning of time. And Homura saw her loved one die in front of her eyes, having to kill her herself 100+ times.
Yuuki Yuuna was certainly dark but there was no tragedy other than Tougo's poor past. Madoka was hardly anything but tragedy except for the end of the anime series and a glimpse of it in the end of Rebellion. But I see where you are coming from.
Also Tougo and Yuuna are practically head over heals for each other, but I think Karin doesn't really have a someone special yet. She fights hard because of Yuuna, but not really sure it's anything romantic. Creators seem to be edging toward FuuRin for sure though.