So there's this manga where monster are sealed into cards and are summoned to fight other monsters/people. Sort of like Yu-Gi-Oh, but not. Here's what I remember: The MC ends up in a lab somehow with many others. They each have a card with a monster inside of it (some weak, some ridiculously OP) with them and are encouraged to fight the death, take the loser's cards, and build a deck. The MC's iconic card is the head of an extremely powerful, evil monster. The monster was split up into 5 cards because it kept killing its user. The MC does not start out with this card. The previous owner of the card got drunk on its power and was swiftly killed by the monster for being 'inadequate'. Because the MC was the true master of the evil monster's card (and I think was always supposed to be), the experiment quickly goes out of hand. The MC joins up with a girl from his school and a little girl to take everybody down and destroy the secret lab. At the end, they have to fight the lab director who possesses the second card of the evil monster (its legs). I think the lab director went slightly insane because the legs card wanted to join up its true master (the MC). The MC and gang run off (I think the MC lost consciousness after the battle) from the destroyed lab and return to civilization before the lab's reinforcements arrive. In the second arc, it is discovered there are more than just the monster-type card secret society. The main enemy in the second arc are machine-type card users. Turns out the reason the MC and co. were taken to the secret lab before was because they were found to have affinity with monster-type cards. They were supposed to be trained as soldiers, but the lab director went insane... so, whoops! They fight the peons of the machine-type group, but I believe those were just the dregs who were given cards and nothing else. I can't remember why they did that though, to sow chaos or something? I know they have a master plan, but I don't think it appeared in the second season. The MC finds a master and gets gud. Turns out the MC has affinity with multiple types of cards (because his head didn't explode when he summoned a machine-type card). The MC finds and beats the holder of the third card of the evil monster dude (the torso/stomach). This guy was also being controlled by the card, like the lab director but even worse. There seems to be a theme of the pieces taking over their host and taking their host's ego for themselves to make itself 'complete' (since the Head possesses all of the memories and original ego).
I forgot how much I liked the girl in this one, she really just rips into people + she has one of my favorite monsters. Pokapoka is such a cute, stupid, and derpy looking monster! But its abilities are so diabolical. Spoiler: Pokapoka