Alright, unlocking costumes is kind of tricky. Every character has two different methods for unlocking costumes. An archaic and time wasting method I'll refer to as the "main" method, and a second method that is almost universally faster, which I'll call the "secondary" method. Once you've met the requirements, you'll have a special battle at one of the arenas against random characters in a random match type. One of the opposing characters will have the outfit you are trying to unlock.
There are seven costumes that must be unlocked before they can be bought, the rest are unlocked from the start. Swimsuits are an exception, and will be covered later.
The first five outfits...
Main
Olga's
Eva's
Nyami's
Mimi's
have different unlock conditions then the two marine gift suits. For all characters, the main method differs for each costume, from winning 10/15 matches against certain characters, or playing a certain type of match with certain characters. I am here to tell you, from experience the main method sucks. It will take you vast amounts of time. I did it the main way and it took over 100 hours and I still didn't have everyone unlocked. The matches are random, so the odds of getting Reiko and Evil Rose versus Anesthesia in a 2v1 handicap match are not good. You will waste a lot of time just trying to get the matches to show up, much less playing them.
Hence the secondary method. For the ten starting characters...
Dixie Clements
Reiko Hinomoto
Aigle
Makoto
Miss Spencer
Candy Cane
Anesthesia
Benikage
Evil Rose
Aisha
you can unlock costumes by starting the game up a certain number of times. You'll need to start up the game 245 times and fight the subsequent matches to unlock all of them. It sounds daunting, but I promise you it's much quicker. In order to register a start up, do this...
1. Load the game
2. Go to the locker room and save. There, you saved a start up. If you have a special battle at one of the arenas, play it before you save, obviously.
3. Exit the locker room and then exit back to the title screen.
Repeat 245 times. Each start up takes about 45 seconds, so you are looking at just over 3 hours. You'll see special matches show up on the arenas, you can play them as they come or you can let them build up and do them all at once. This seems to be slightly glitched, sometimes the special battles won't show up until you play the character that the outfit belongs to, so don't worry, you haven't lost your chance.
The marine gift suits are a different story. The main requirement for them is to use every penalty in a queen's match. In order to use it, you have to pick it before the battle and then win the match. Some of them you have to buy. The secondary method is playing a certain amount of matches. Reiko needs the least amount for the secondary method, 50 matches. There are something like 26 penalties, so the main method is faster here. Use a checklist so you know which you've used already.
The main method for the first 5 suits for the unlockable characters...
Sgt. Clements
Sista A
Dr. Anesthesia
Becky
Noble Rose
Yasha
BBD
Great Khan
Rowdy Reiko
Mistress
is the same way. Ridiculous time wasting requirements. The secondary method is different for unlockable characters though, you have to play a certain amount of matches. Every unlockable character has to win 80 matches to unlock all suits, and the two marine gift suits take 70 and 80 matches total.
So what we do here is use every queen's match penalty. This gets us the two marine gift suits, and also gives us around 26 matches played. You need only win 34 more for the rest of the 5 other suits. For the last 34 matches, you can do tag team matches and get money for two characters, and this is what I recommend. Make your tag team partner an edit character with the "shooter 1" moveset. Find a tag team match, then tag the edit wrestler in with right trigger. Then press Y + right to grab your enemy, then Y + down to stomach punch them thrice, and then Y + left to do a punch kick combo, then pin.
Occassionally, the enemy tag partner will rescue her ally. If that happens, let your enemy get up, then throw her against the turnbuckle near her tag partner. This will knock the tag partner off the ring, and stun your enemy. Killer move her and then pin for the win.
Alright, now that that is out of the way, swimsuits. These are really easy. Have a character wear her swimsuit, then win a match in each of the three arenas. It will then be available to all characters for purchase. If only the rest was that simple, right?
I understand this has become a gigantic wall of text. If anything is unclear, you can comment here, or PM me directly. I'm on everyday and will answer questions ASAP.