Looking for a series that has a mysterious tone to it.
It can be that the character is mysterious or something strange is going on but I want there to be a satisfying romance not something open ended. Coelacanth is a good example of that.
I don't want anything supernatural or murder sprees. Just a very slice of life mystery.

Uh, wow, your parameters make this kinda tough....a mystery but it can't involve violence (murder) or anything supernatural, plus it's a mystery that's going to have a "satisfying" romance.......not "open ended".

Geez, I think the only one I can recommend that might still fit into that would be Dengeki Daisy.

Good luck with it

Yeah, it's rare. mystery without supernatural and murder thingy is already rare, moreover with good romance.

Daddy long legs, almost like Dengeki Daisy, female lead is supported by mysterious person. The romance is good, but

Aoba-kun ni Kikitai koto, about a girl who trying to solve her crush's problem, without the boy wants to tell what the problem is

Nanairo Kakumei, not really mystery actually, but you have to think a lot to understand and enjoy the manga without flipping any table.

Ok so, I have one, called Madame Petit, but there's only like 3 chapters translated and so far the romance is one sided. Its a historical manga. Usotoki Rhetoric might also fit, but the romance isn't all there (also historical). And well, there's a murder case, but it's good. And maybe Usemono Yado . Its complete. I'll warn you though, this one has tragedy (its set in a hotel/inn with flash backs and thing)... I may have more, so maybe I'll come back and edit this post. But those are the ones off the top of my head.

[https://www.mangaupdates.com/series.html?id=511] - Shingetsutan Tsukihime definitely recommend this

Thankssssss for the suggestionssss
must have morreeeee