He MUST be possessed at one point. Otherwise, I'll never see my lovely true king of dragons, RE Darkness Dragon, or its Metal form in DL, like EVA!!! D:
He was possessed once, and the other 99% of the time he was not. He also could simply choose to be nightshroud anytime he wanted, so he was only really possessed in the shadow riders arc.
You make it sound like Atticus was around for the whole GX season, which no, he was only around for MAYBE half of it and even that was 60/10/30: Nightshroud vs Comatose vs Overbearing big bro. The other 40% he never really dueled so the only cards he's most known for are Darkness Dragon & CyberDarks, his Nightshroud cards. (I'm hearing Atmosphere too but no one cares)
in the Anime, his normal deck (when he's not being Nightshroud) is implied to be a Normal Monster Beatdown Deck with Red-Eyes as his ace. Though we only saw his regular deck once during one of the later seasons.
Outside of events, we're probably getting that version of him with a bunch of Normal Monster deck support. and Maybe Darkness Dragon.