Like the title says, i want a series where the protagonist knows people from different groups. Example:

1. Akame ga Kiru! where Tatsumi works for one side while having a romantic relationship (albeit one-sided love) with a leader from the opposite side. However, it doesn't have to be strictly like that.

2.Aiki where, in recent chapters, Joukyuu pretended to join the opposing side and betrays them later.

Different scenarios are perfectly acceptable as long as the main requirement is fulfilled. It's just that I can only remember these two mangas from the top of my head.

Thanks in advance

Vampire Juujikai perfectly fits, (not saying anything specific to avoid spoilers). Edit: I missed it in your lists, sorry.

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What I like in Trivial Pursuit style. Pick your category:

Superior - demon lord joins a party that is on a quest to kill him (actually her)

I've already read both of them, but these are also good examples. Thx for posting tho. Anymore suggestions?

If I'm not mistaken, because it's really been a while, Jackals might also be worth a try. ^^

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"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

ouroboros would be a good read. it definitely fufills the conection requirment

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Take the Magic: The Gathering 'What Color Are You?' Quiz.

Zetsuen no Tempest is a good one, actually you can't be even sure who is on which side and which side is bad at all. But beware, it is pretty complicated and they talk a lot and long.
Btw, author is the same as Vampire Juujikai's.

Alive: Saishuu Shinkateki Shounen the protagonist is friends with the antagonist, ala Naruto and Sasuke.

Eden: It's an Endless World! i can't remember much, but the protagonist sometimes good, sometimes bad, he can go bothway.

MW the protagonist have a sexual relationship with the antagonist. now, this is NOT yaoi or shounen-ai, there is no such thing as romance here.

Superior Day the connection is not revealed till the end...it's very interesting how they are connected.

Usotsuki Mi-kun to Kowareta Ma-chan Totteoki no Uso the protagonist know all the principal characters.

Shin Angyo Onshi part of the protagonist past

Hakushaku Cain Series the big bad is the protagonist's father.

they are all completely scantalated, so dig in!

I love these (except i like it better when they in love ^.^)

Basara i love this one
7 Seeds not sure, but give it a try

I didn't really like Alive. It was too childish. I've also already read Shin Angyo Onshi. The rest of the suggestions look nice tho. Thanks for answering!