I didn't knew how to put it in the title but.. I'm searching for a manga with a main character that is or becomes a enemy of humans in general, someone or something that as soon as its true form is seen by a human, it makes them either run the hell away or try to kill him/her.

It can be someone like a serial killer or a monster that feeds on human flesh, what's important is that he/she enjoys it.

Also that character must develop a non-hostile relationship with a human, it can be either love, friendship or anything like that.

I can't really thing of many examples right now but... something like this

Well, I'd say Hellsing fits to the description of what you are asking after.

The main character is a terribly powerful vampire who takes immense relish in slaughtering humans. He is also in direct servitude to a human with whom he has quite a strange relationship.

[img]http://i2.kym-cdn.com/photos/images/original/000/028/438/5700.necropost4kobf7.png[/img]

hmm...interesting, I like where you are going with this.

Kemonotachi no Yoru - Fits the best of all the ones I can think of. Sakura is possessed by the demon Kagara (at times) and Kagara wants to kill all the descendants of Mikage. Whenever one of these descendants sees Sakura/Kagara, they freak out either trying to get away or trying to kill him. Later on, Kagara doesn't want to kill Aria (a descendant of Mikage and the main heroine)

These ones kinda fit. Doesn't hurt to check 'em out.

Trinity Blood- It seems he used to be like that before the events of this story. He's kinda ashamed of it now tho, but he has his scary moments still. The Methuselah (vampires basically) as a whole fit your specifications at times too.

Vampire Juujikai - well it seems that way sometimes... but then it doesn't and the it does. nothing is as it seems in this manga. It makes you question yourself as to what is actually true many times.

Demon Seiten - I've read only a volume or so, but it does have that premise (demons cause people to age backwards until they disappear except for the people that can make contracts with them).

Hell baby XD although I don't think she enjoys it, and never read enough to know if it gets its long awaited human companion.

:Reversal definately has the hideous creatures and the human connection.

Fourteen hilariously deformed creature though not sure about the human connection again.

Theres also a bit in Demon flowers where a girl sympathises with some pretty nasty looking demons...

wolf guy - wolfen crest has the human connection but he can transform into it(werewolf obvious from the title lol) and he gets the human connection before she sees the beast side...so it doesn't entirely fit.

There's a character in The Voynich hotel that looks like a zombie and a human falls in love with them. Not the main couple or anything though.

Also if you're looking for something like pupa then Raiders is very similar, but doesn't have the transformation into monsters, its more the cannibalism side thats similar.

Superior MC enjoys killing humans at the beginning but it changes later.

[img]http://www.mediafire.com/convkey/a8ee/v1c0uzwpt6cdtcbfg.jpg[/img]

Pet shop of horrors

I am the master of my fate;
I am the captin of my soul.
William Ernest Henely

I rate manga with 5 being the average score.

Have you read Parasyte?