Marona is a World Eater like the main character in Soul Nomad.Marona can absorb phantoms to make herself stronger. World Eaters can eat souls to make themselves stronger. Therefore Marona is a world eater.
Fusing Phantoms (or banishing them) does not destroy their souls.It just uses the psychic...stuff (it's not ectoplasm) that Marona infuses them with for manifestation. Upon giving it back to her; they float around intangibly or invisibly until she "creates" them again at level 1.
Phantoms are not the souls of dead people, rather they are psychic copies.This is why Marona can "capture" the same weapon from opponents multiple times, and they'll still have them in cutscenes.
Alternatively, Marona is the Dimension Lord of Ivoire, or at least her island.A Demon Overlord without the "Demon" part, she can give any dead soul an "alternative" afterlife to being a Prinny. (Something they even comment on.) She has all the powers of a Demon Overlord, especially apparent if one uses Level Grinding to engage in post-Endgame stages. Whatever Powers That Be that govern Ivoire don't mind her, as it's obvious she's not out to challenge them.
Marona is not as innocent as she acts.Look at what happens to her in Disgaea 3, after Baal supposedly stole her innocence. (I say supposedly, because after you beat him/her/it none of the lost/missing/stolen things get returned and Baal denies the theft.) Plus Marona is effectively a Necromancer—yes, in the Nippon Ichi world Dark Is Not Evil, but there's no way Marona's that naive, especially considering the treatment Ivoire's natives give her. She just that determined to remain an optimist.
Marona's grimness in Disgaea 3 is due to massive psychic trauma.After she kills Baal; he forcibly joins her party as a Phantom. According to Makai Kingdom, whenever Baal's body dies, he immediately possesses someone else's. He tried to do this with Marona but she's already the Possessed One; it didn't affect her the same way it would someone else. So Marona has THE Lord of Terror Supreme Demon Overlord's soul inside her. ....yeah. He gathers enough power to try and take over again; Marona banishes him, but he pulls part of her soul out with him and drags her physically to Mao's underworld.
Marona grows up to be Adell's mother from Disgaea 2.After all her friends get sent away or lost by various means, she goes to look for them, time differences in flow between dimensions prompting the timeline to look really weird. At some point, she stops in Veldime after having fallen in love, and likely given up on finding her friends after literally decades of searching. Then the False Zenon comes and casts his spell, and she forgets all of her past, but not her powerful summoning skills. Cue plot of Disgaea2. What she does afterwards, when her memory is restored, is up for debate, but given that in the Updated Re-release she can encounter Ash and her younger self...
After Sulphur was beaten as a Bonus Boss, the Merchant of Death captured him.The original story, let's call "Multiverse A". Marona and company shove Sulphur into a Dimensional Gate and shut the door behind him; forcing all his essence into one point. Lujei Piche finds him in the Void between Dimensions and sends him back for giggles, then Marona kills all of him. He still exists, but is massively weakened, and his core is stolen by the Merchant of Death using demonic arts. The Merchant of Death uses him to threaten Carona's World; let's call that "Multiverse B." Carona fights it; and the Merchant of Death is impressed enough that he enslaves her on threat of reviving Sulphur again and goes to find another Marona. This is the plot of Another Marona; and we'll call this time split "Multiverse C." After the events Another Marona, Carona is on a quest to destroy the Magenta Core; which is Sulphur's Soul Jar.
Another Marona isn't just an Alternate Universe from the main plotline, it's a changed Timeline.The Magenta Core doesn't just open doors in space, but time too. The Merchant of Death unleashes Sulphur in weaker forms in the worlds, and when someone beats it; goes back to before they get strong enough to do so and makes them his slave. He didn't have this function when Scarlet the Brave first beat Sulphur.
The Pringer X series was created by the Merchant Of DeathSounds like something he would do, doesn't it? In fact, he pulled the same stunt as with Sulphur, only this time with Baal. After Marona defeats Baal; the Merchant of Death used Baal's soul energy to animate his cyborg monstrocities.
Fishcakes in Another Marona are dessicated corpses.We know some bodies survived that get Sulphur Shadows confined into them. And suddenly in an Empty Ivoire, there are these everywhere....
The people you murder in the main storyline are the Phantoms you can eventually create with Marona.Considering you can't create a human character until you kill one somewhere, Marona could very well be using the very people she killed before.
The Merchant didn't technically resurrect the people he killed; he Perma-Confined them.The same power that Zetta has from Makai Kingdom; the Merchant's Magenta Core can do it to the air. Hence the bodies not being required (and getting killed by the cast)
The Merchant of Death was behind breaking Nemo.This one is convoluted, but bear with me.