Capcom vs. SNK/Secrets

Enable "Extra" mode characters
Press and hold the start button while selecting a characters

Pair Match Mode in the arcade
At the title screen, press and hold the and  buttons, then press the Start button. All characters will be Ratio 2, and you may select two of the same character.

Fight Morrigan or Nakoruru
To fight against Morrigan or Nakoruru you must earn at least 60 Groove Points before your fourth fight (in either Arcade Mode or Pair Match Mode). If you selected Capcom Groove, you will face Morrigan. If you selected SNK Groove, you will face Nakoruru. (On the Sega Dreamcast version, you must first buy their shadows from the Secret Shop, numbers 63 and 64.)

Fight Akuma
To fight against Akuma you must earn at least 80 Groove Points before your final fight. (On the Sega Dreamcast version, you must first buy his shadow from the Secret Shop, number 62.)

Alternate Versus Points method
The ability to link the Sega Dreamcast version of Capcom vs. SNK with the Neo Geo Pocket Color version of SNK vs. Capcom: Match of the Millenium allows you to transfer Versus Points back and forth between the two games. It just so happens that there is a way to get 5000 points in SvC: MotM instantly, but you must have access to a second NGPC, and a copy of SNK vs. Capcom: Card Fighters Clash with all 300 cards available. If you link a completed game of SvC: CFC to SvC: MotM, you will unlock every hidden fighter and gain 5000 vs. Points. You may then transfer those points to the CvS. Then erase the save file in SvC: MotM and link it to SvC: CFC to get those 5000 points back, and repeat the process.