SECRET TEAMS
There are 2 secret teams in the game:
The Yumemi Team : To unlock, simply beat Single Play once
Wu Wonfu, the boss team!: To unlock, beat 5 stages of Endless Play

SECRET CHARACTERS
Now, secret characters are a different deal, they are unlocked instead, by beating Single Play with a combination of Teams; some combinations unlock 1 character, others unlock 2 or more.
Anyway, there are 18 secret characters in total:

Legend & Japan All Stars
Toraichi and Torazi, the Tiger Brothers WITHOUT their masks! :
Kagemura High School & Kirisame High School
Himada, the Extreme Cheer!:
Reiho High School, Hattori High School, Reiho Sports Club & Reiho's Top 4
Ryuichi and Ryuji with their other design! and Tsukasa, Mochizuki's younger brother!:
Akiba High School & Tobioka CC
Kinjishi, the millionaire student CEO and the american robot designer Tobioka!:
Nekketsu Dodgeball Club, All US & International All Stars
Moldov, the Russian Abobo, captain of the Russian Dodgeball Team!:
Outlaw, Team Misuzu & Lion Union
Shinji, the Emperor Blue, boss of the motorcycle gang!:
School Union & Historical Drama
Torazo, Kumada's historical drama counterpart & Nedachi, the Historical Drama fan!:
Senridai High School & Historical Drama
Sankichi, Nedachi's historical drama counterpart!:
Reiho Former Student Council & Historical Drama
Jinroku, Onizuka's historical counterpart!:
Reihou Former Student Council & Meian High School
Iwaki!:
Hanazono High School, Yurigaoka Girls's High School & Cheerleader
Toru, the silent swordsman & Bob! :

Finally, the single character with a double unlock condition:
Munakata Yuuji, sole heir to Munakata's Industries and the other Final Boss, besides Nizaemon!
To unlock him, you either need to beat Single Play with all 30 Teams (Which isn't that much considering you already need to win with around 23 teams in the first place to unlock the other characters) or alternatively, beat 10 rounds of Endless Battle (For this, I recommend either Shingo from Tobioka, Hasebe from Cheerleader or any character from Nekketsu Magic Story, assuming you have hands)

http:// www.media fire.com / file / 6c0mx7x5fz8wucn/SAVE00.BIN
(Remember to remove the spaces)

To use it, simply go to the game's files --> USDIR --> SAVEDATA
And replace it