I copied and pasted this from someone:
"i think, the mana that acted as a father to allen before was the millenium earl when he was still only one.. and when he was separated into two, he bacame mana and nea.. the new mana is the one who ate nea and nea resides within allen since allen is related to the first millenium earl mana... now it gets more confusing"
I recommend reading the wikia pages for D.Gray-Man. That might help explain and clarify a number of things. However, there are still many questions and mysteries left to be answered and solved in the story, so that may be adding to the confusion too (like why/how Red became Nea's host when he isn't the real Allen, or what/who is the Heart and is the Heart even a good thing? And what are the Noah exactly? And who is the puppet master? Is it the real Allen?) So many questions and so few releases ╥﹏╥
Who is red?
Our Allen before Mana started him called "Allen". He got the name Red from his arm I think. "Allen" isn't Allen's original name.
Red was the name Allen went by before meeting Mana
Okay I've read this manga like 2 times now and of course the animes but I still have no idea what's actually going on. Don't get me wrong though, I still love it to high hell, but my tiny ass brain can't keep up lmao