Capcom does not make games to explain things. Every game makes it more difficult to understand what the hell happened.
DMC1 facts:
1) Sparda rebelled against Mundus and sealed him along with the rest of the demons.
2) Sparda came in Earth, married Eva and had twin children named Vergil and Dante.
3) Sparda dies(?).
4) Mundus kills Eva directly or indirectly.
5) Vergil disappears, is though dead by Dante.
6) Dante opens Devil May Cry to slay demons and find the one responsible for the death of his mother and brother.
7) Dante uses Ebony & Ivory and Force Edge, a memento of his father.
8) Dante wears an amulet, gift from his dead mother.
9) Mundus resurrects and attempts to reopen the gate.
10) Mundus sends Trish to lure Dante into a trap.
11) Trish succeeds.
12) Dante kills various devils and amongst them Nelo Angelo, who had an amulet identical to Dante's and use almost the same sword attacks as him.
13) Mundus reveals that Nelo Angelo is/was Vergil.
13a) Dante combines the two amulets and Force Edge, and it transforms to Sparda, the ultimate sword that his father used.
13b) It is revealed that the amulets as one are the key to the underworld.
14) Trish tries to kill Dante.
15) She fails and get rescued instead.
16) Dante meets Mundus.
17) Mundus uses Trish as bait to kill Dante.
18) Trish sacrifices herself to save Dante.
19) Dante beats Mundus.
20) Dante leaves Sparda and the amulet to Trish.
21) The island is nearing an explosion.
22) Mundus returns to attack Dante.
23) Trish is revealed to be alive and gives Dante enough power to beat Mundus.
24) Dante and Trish escape the island.
25) Dante and Trish live/work together in Dante's shop, now called "Devils Never Cry".
26) Trish uses Sparda for the job.
After that there was DMC2 who had these questions:
1) Where is Trish?
2) Dante wields Rebellion instead of Force Edge/Sparda?
3) Sparda fought demons in Earth too alongside humans.
4) Dante does not have his amulet.
5) The gate, opens with some mysterious relics called Arcana instead of the amulet.
6) Bolverk, a demon supposedly fought Sparda. However he is not the most difficult demon to fight.
7) Argosax was demon emperor.
8) Where is Mundus?
Then DMC3 came who gave us new questions:
1) Vergil is still in Earth?
2) Dante uses Rebellion, and apparently now is a sword that Sparda gave to Dante.
3) Vergil's amulet is suddenly gold instead of being an exact copy of Dante's amulet.
4) They way to open the gate to the underworld is changed again. This time, it still needs the two amulets but also the blood of a pure, innocent girl (a priestess?)
5) Sparda sealed his own powers as well.
6) Force Edge is inside the demon world.
7) The two amulets along with Force Edge, did not create the sword Sparda as in DMC1. Could it be that you need Sparda blood to awaken it? It is never explained.
(Of course DMC3 had some answers. It showed us the method Sparda used to close the underworld, how Vergil met Mundus, how Dante opened Devil May Cry and how he got the Force Edge)
DMC4 is the best mystery of all:
1) Nero has the blood of Sparda(at least that what a crazy old man said), however not one person knows how he got it, and apparently not one person cares about it either.
2) Lady in DMC3 was like a zealot who wanted to kill every last demon herself. However, now she doesn't really care and offers the job to Dante instead.
3) Yamato reappears out of nowhere(?)!
4) Yamato appears to be a key for a Hell Gate that Sparda once sealed.(?)
5) Yamato broke. (?)
6) Nero has Vergil characteristics. (?)
7) Sparda served as a feudal lord of Fortuna long ago(?).
My point is that instead of a story about Sparda, it is more likely we'll see something new that we don't even expect, probably adding to our questions
