You get secret characters by winning the regular arcade series while playing with each of the various characters. Once you get them all, move your cursor to the bottom right of the list of characters and you'll get the Unknown Girl. She randomly does moves of the other characters - sort of like the polygon team in Super Smash Brothers. You can tell which she is by her victory stance and the moves she makes. To the left of that same list is the wood/metal flower-head Tetsujin.

Also, you can hit 'start' on Eddie to get Tiger, or start on the devil to get the angel. Normally the 'start' button just changes clothes.

Note that the Angel/devil light blasts make jack series go into blast arm attack

Stages 1-6 rotate randomly, but it seems that Stage 7 involves people related to your combatants. Stage 8 is always the unknown person.

Winning with the various characters also unlocks cool video sequences which don't really mean anything, but are fun to watch.

Tekken Tag Team Walkthrough