@KZO 
1. Oh please, don't play psychologist with me. Why would you randomly pair a creature that hasn't appeared in this manga yet alongside species that did? We both know well enough that the fandom wrongly calls Shamiko a succubus at every turn. This has nothing to do with what I think she is, it's simply a pretty obvious implication from you. And now that you revealed that you said it to bait exactly that obvious counter, it just proves that you indirectly were talking about the Yoshida family. 
Why do you insist on making yourself look like a fool?
2. The factor in question is being abnormal/a threat. That is unrelated to any physical, lifespan or ability related factors. So you have no argument on them being similar in any other aspect. Again. Nothing more to say to your bad argumentation.
Of course I already knew you were just parroting /a/. I recognize many of those half-assed head-canons from a mile away. I've been around /a/ long enough to get sick and tired of them.
@shaeli
That's actually an interesting point. Difference in lifespans always has a tragic aspect to it when you get romantically involved. Although I seriously don't think he has any interest in her that way.
@MidnighTea
I'm not too sure about that. After all at first Riko treated Momo like an enemy and tried to trick her into falling into traps and didnt want to interact with her at all. She started to like Momo when she realized how easy she is to tease and how despite her bad attitude she is kind to Dark Clan, as 
@thofheinz mentioned. They are also kind of reflections of each other, so Riko probably feels like she has found a kindred spirit. I think Riko's gratitude all flowed into Shirosawa rather than Sakura.
Though yeah, Riko's support is being trampled by Momo's dumb boyfriend denseness. lol