In the form of a What the Hell, Hero? for Sparda, several fans have wondered why exactly Sparda didn't leave his family with the Vie de Marli for safekeeping considering that he helped them kill an evil demon god (which means that they probably owed him a favor) and that they were more than capable of warding off some measly demons sent by Mundus. The potential problem is never addressed in-game.
Considering Sparda's age, it's quite possible that he helped the Vie de Marli decades before meeting Eva. We also don't know what happened to Sparda, so maybe he planned to do it but was killed before he could.
The Vie de Marli could fend off common mooks, but what if more powerful demons were sent, or worse Mundus himself paid them a visit, like in the reboot?
The series' Central Theme about how one's humanity is a source of strength and not weakness is displayed in Lucia's battles with Arius in 2. When Lucia first fights Arius, she has recently learned that she is one of his artificially-created demons and is terrified that she will become a threat to the people she's supposed to be protecting. When Arius defeats her, she even tries to goad him into killing her. When Lucia fights Arius a second time, she has reconciled with her origins while Arius has returned as a twisted demon. Lucia is at her strongest when she views herself as a person instead of a tool created by Arius, whereas Arius sacrifices his intelligence and versatile magic for demonic power he has no control over and he is defeated.
Arius is able to tell Lucia apart from the other Secretary demons despite her looking exactly like them. This ironically shows that Lucia is more human than she thinks as even Arius can tell the difference between her and the other artificially created demons who look like her. Likewise, his fights against Lucia show he's at his most dangerous as a human. He easily defeats Lucia, outsmarts Dante, took over the entire island and his Summon Magic was at its peak. As a demon, Arius is reduced to a clumsy brute that has to rely on projectile spam, goofily belly flopping on the ground, and degenerates even further into a mindless lizard.
Dante picking up his Heads or Tails? quirk in DMC2 may seem strange, using his coin several times throughout the game, before it's revealed that it's a trick coin where both sides are heads. However, in a bit of a Call-Forward, Dante in the first episode of TAS promptly picked heads when Morrison challenged him to a coin toss on whether or not to take the job of being Patty's bodyguard. He lost the bet with Morrison (it was tails), and it's been stated that he's never been that lucky when it comes to gambling, hence the quirk seen in 2.
The reveal that the coin has two heads can be seen as Dante wanting to "make his own luck" and assert that he is in control as opposed to every other game where he is duped or very unlucky.
The North American instruction manual states that the Protectors of Vie De Marli could call upon the powers of their gods to vanquish their enemies. This ability is never seen in the game proper were Lucia describes the Protectors as humans with demon blood. While this is likely an issue with translation and Devil May Cry 2 being a rushed game, there is a way to reconcile these two supposed inconsistencies. In Japan, Vie De Marli is referred to as "The Island Where Holy and Evil Merge". In short, the Protectors clan could be made up of demon-human hybrids, humans who can invoke the powers of divine entities and even those who are both. This is exemplified in Lucia, an artificial demon created by an evil human who nonetheless fights on the side of good and has an angelic Devil Trigger form.
