
Psychopath/Assassin/murderer gets involved with female lead

It's Yuri, it that's okay? Here are a couple I've read.
- Vengeance
http://www.mangago.zone/read-manga/vengeance/
- Serenade
http://www.mangago.zone/read-manga/serenade_keum_kyesoo/
(official links)
https://www.lezhin.com/en/comic/vengeance_en
https://www.lezhin.com/en/comic/serenade_en

Shounen/Seinen with a female lead who has a destructive strong power and hides it or doesn't know about it?
Examples:
Deadman Wonderland
Brave 10
Code Breaker
Rave Master

i have something that is similar
http://www.mangago.zone/read-manga/the_strongest_girl/

Happy-Go-lucky female lead with a dark past?
Like in Deadman wonderland or Code Breaker.

Fruits Basket. A sad past, not neccessarily dark. A classic.
http://www.mangago.zone/read-manga/fruits_basket/
Shounen/Seinen manga with best love story?
http://www.mangago.zone/read-manga/psyren/
http://www.mangago.zone/read-manga/rave_master/
Sadly I've read both of them already
Seinen - Sekine-kun's love. HANDS DOWN. I've never cheered up for a manga character's love life so badly than him.
shounen - nisekoi (this is harem tho but i am into harem) i actually really like this, the concept of "sawaze in love" key-thing is romantic.