There are a total of 40 Characters (Fighters) in the starting roster (not inclucing DLC) which you will become unlocked once you have cleared the prologue of the game.
After viewing some cutscenes you will need to create your own fighter and go through a tutorial fight that will take you to the hub area where you will be able to start missions, play offline and online battles. At the center of the hub area there are 3 tunnels that go in different directions, at the end of each of these tunnels you will find one team leader. You must talk to all 3 of them.
Goku
The middle tunnel in the hub area is the red 'A' tunnel which will take you to a desert area where you can talk to Goku.
Luffy
The left side tunnel in the hub area is the blue 'B' tunnel which will lead you to an island with a ship docked there. Here you will find Luffy.
Naruto
The right side tunnel is the the green 'R' tunnel which will take you to a hidden leaf village section where you can talk to Naruto.
Director Glover
Once you have spoken to each of the team leaders you go through the fourth tunnel which is also reachable from hub area and is found on the opposite side of the red 'A' tunnel. This tunnel will take you to an Operations Office wherer you can talk to Director Glover. This will be followed by a combat tutorial which once completed will immediately unlock all the characters.
There is no character progression, you just need to complete the little prologue tutorial. Doing so will unlock the mission counter and enable you to interact with the shop and rewards vendor. Below is a list of all 40 characters along with the franchise they come from).
• Asta (Black Clover)
• Renji (Bleach)
• Ryo (City Hunter)
• Trunks (Dragon Ball Z)
• Piccolo (Dragon Ball Z)
• Cell (Dragon Ball Z)
• Kenshiro (Fist of the North Star)
• Boa Hancock (One Piece)
• Himura Kenshin (Rurouni Kenshin)
• Shishio Makoto (Rurouni Kenshin)
• Dragon Shiryu (Saint Seiya)
• Pegasus Seiya (Saint Seiya)
• Toguro (Yu Yu Hakusho)
• Yuske Urameshi (Yu Yu Hakusho)
• Kaguya Otsutsuki (Naruto Shippuden)
• Kakashi Hatake (Naruto)
• Gaara (Naruto)
• Bortuo (Boruto)
• Jotaro Kujo (JoJo’s Bizarre Adventure)
• Dio (JoJo’s Bizarre Adventure)
• Dai (Dragon Quest)
• Izuki Midoriya (My Hero Academia)