Yukari is so cool! I mean, purely aesthetically speaking, she's already got so much going for her! Her gap powers are distinct and interesting. You can do so much with just that alone! Do they lead to some eldritch realm? Is she some kind of horror from beyond the veil? Or if you're sticking to comedic styled themes, she's just a crazy-old youkai that likes dicking around all day. Her past, her powers, her entire being is ripe with the potential for stories and intrigue!
Ran is an oft-overlooked character whose intrigue mostly comes from her relationship with Yukari; what brought these two together? A lot of really interesting doujins go into that. Of course, she's also a rather heartwarming character. Fanon or not, seeing her dote on her little youkai underling is adorable!
Chen, of course, needs no introduction! Those strangely humorous comics written by that artist whose name I can't FUCKING REMEMBER GODDAMNIT have all but guaranteed her place in the spotlight! Mostly she's just used for light hearted antics, but she's got a serious side to her, too! Although I haven't read any doujins depicting such lately, so, that's unfortunate
Of course, the Yakumo family as a whole proves to be amazing in tons of different ways. Heartwarming, terrifying, calculating, they can be used in tons of ways!
My personal favorite depiction of them is when they're being cute. Ran doting on Chen, Yukari lazily screwing around, and more make this family really memorable