Change various parts of characters:
Select a character, but instead of pressing A again, press X, Y, or Start and it will show the character (it will not show the changes). Now press A again to start the match.

Note: this changes mainly face related things, like hair style, hats, masks, etc. Not all characters have three different things, or necessarily any changes at all.

Unlockable Characters:
Ein Beat Story mode with Hitomi
Gen Fu Beat Story mode with Eliot
Leon Beat Story mode with Zack
Helena Beat Story mode with all 16 characters
Nicole (Spartan) Beat Story mode with Helena
Tengu Beat Time attack for all characters, including unlockables

Unlockable Costumes:
To get a costume for character that can be used in the story mode(16 default + Helena), beat storymode with that character once for a costume. For character that can't be used in story mode, simply beat Time Attack mode [with default settings] instead. Difficulty doesn't matter.

Kasumi C3-C7 Beat Story mode with her one time per costume.

Kokoro C3-C7 Beat Story mode with her one time per costume.

Hitomi C3-C8 Beat Story mode with her one time per costume.

Helena C3-C5 Beat Story mode with her one time per costume.

Leifang C3-C7 Beat Story mode with her one time per costume.

Tina C3-C5 Beat Story mode with her one time per costume.

La Mariposa C3-C5 Beat Story mode with her one time per costume.

Ayane C3-C6 Beat Story mode with her one time per costume.

Christie C3-C4 Beat Story mode with her one time per costume.

Spartan-458 C3-C7 Beat Time Attack mode with her one time per costume.

Hayabusa C3-C4 Beat Story mode with him one time per costume.

Elliot C3-C4 Beat Story mode with him one time per costume.

Brad Wong C3 Beat Story mode with him one time per costume.

Bayman C3 Beat Story mode with him one time per costume.

Bass C3-C4 Beat Story mode with him one time per costume.

Zack C3-C4 Beat Story mode with him one time per costume.

Jann Lee C3-C4 Beat Story mode with him one time per costume.

Hayate C3-C4 Beat Story mode with him one time per costume.

Ein C3-C4 Beat Time Attack mode with him one time per costume.

Genfu C3-C4 Beat Time Attack mode with him one time per costume.

Leon C3-C4 Beat Time Attack mode with him one time per costume.

System Voices:
Each character has his/her own voice for the system. To unlock the particular characters system voice, you must complete their excercise in sparring mode.

Hidden Stage:
Sparring stage, floor mat with no walls.
Nassau Station Becomes available once Nicole (Spartan-458) is unlocked

Theater Mode:
Complete "Story Mode" once using any character to unlock "Theater Mode". The movies for each characters will be unlock once you finish their story.

Unlock Tatami Stage:
Get all costumes and complete each character's exercise (including unlockables) to get access to the Tatami stage for use outside of Sparring mode.

All secret achievements:
This is a list of all 15 secret achievements and how to unluck them in Dead or Alive 4.

0 Gamerscore Points Lose 5 online matches in a row.

0 Gamerscore Points Lose 10 online matches in a row.

0 Gamerscore Points Lost 20 online matches in a row.

0 Gamerscore Points Achieve a grade "D" in DoA4 online.

0 Gamerscore Points Achieve a grade "E" in DoA4 online.

Achieve a Bronze Star. 10 Gamerscore Points.

Beat someone with a 10-14 win streak in DoA4 online.

Achieve a Silver Star. 20 Gamerscore Points.

Beat someone with a 15-19 win streak in DoA4 online.

Achieve a Gold Star. 30 Gamerscore Points.

Beat someone with a 20+ win streak in DoA4 online.

Unlock Leon. 10 Gamerscore Points. Complete story mode with Zack.

Unlock Gen Fu. 10 Gamerscore Points. Complete story mode with Eliot.

Unlock Ein. 10 Gamerscore Points. Complete story mode with Hitomi.

Unlock Helena. 20 Gamerscore Points. Complete story mode with everyone.

Unlock Spartan-458. 20 Gamerscore Points. Complete story mode with Helena. Unlock Tengu.

20 Gamerscore Points. Complete Time-attack mode with everyone. That includes secret characters.

Play DOA4 for 5 hours 5 Hours Played (10 Points)

Play DOA4 for 10 hours 10 Hours Played (20 Points)

Play DOA4 for 100 hours 100 Hours Played (40 Points)

Unlock all Costumes (50 gamerpoints) Unlock all costumes for every character (including secret characters)

50 Wins in DOA Online (20 gamerpoints) Win 50 games accumulatively in DOA Online.

Rank B in DOA Online (20 gamerpoints) Win enough DOA Online games to achieve this rank

All Items in Survival mode (25 gamerpoints) Collect all items in Survial mode

Trophies for Time Attack:
Choose time attack either Single or Tag. Note, these can be done with just one round for the win (making it excessively easy).

Gold (Single) Complete single in less than 4:30
Silver (Single) Complete single in 4:30 to 5:00
Bronze (Single) Complete single in 5:00 to 6:00
Gold (Tag) Complete tag in less than 2:30
Silver (Tag) Complete tag in 2:30 to 3:00
Bronze (Tag) Complete tag in 3:00 to 3:30

Trophies in Survival Mode:
Bronze Trophie (Single) Get 20 Wins
Bronze Trophie (Tag) Get 20 Wins
Silver Trophie (Single) Get 50 Wins
Silver Trophie (Tag) Get 50 Wins
Gold Trophie (Single) Get 80 Wins
Gold Trophie (Tag) Get 80 Wins

Warthog in Gambler's Paradise:
When you play as Spartan-458 on the level Gambler's Paradise a Warthog drives down the road at some time during the match carrying UNSC marines hanging off the side.