I'm looking for a manga that if it exists has the following (twisted) element.

Two characters (one male & one female - that look nothing alike), one who is the present incarnation and one who is the former's previous reincarnation (now resurrected), who have since or will eventually fall in love with each other.

It would deal with themes along the lines of the present incarnation's feelings (good and bad) towards the previous reincarnation who effectively created him / her and how the previous reincarnation's action's and personal baggage impact their relationship (such as the previous reincarnation's grown up kid falling for the present incarnation, creating a messed up love triangle).

Sorry if it does not make much sense, its just an element that I have yet to find in a manga.

Well, the weather's looking bad. It's bad here, bad there - bad everywhere. Nasty alien creatures raining down from the skies, devouring all life on this planet. And what does that mean to you, you ask? It means you're mine. You're all mine! - Prince (aka Death) from Lexx

Omae ga Sekai wo Kowashitai nara

is kind of complicated but I think some of the elements you mentioned can be found there...

Ayashi no Ceres
Please Save My Earth <--- really messed
Gekka no Kimi <-- got really bored of it

there are a lot more. just simply click on the reincarnation tag in the genre section

Inuyasha has exactly that, except they are the same gender. But there is still a love triangle between the reincarnation, the former person, and the person they both love.

oh i know fushigi yuugi but no wait ... i got confused with the anime in the end of the series the guy reincarnates in the girls world oh but i know one tonari no shugoshin 🙂