to fight great joe, which if you beat him, you get the third swimsuit item, and also get him as a summon monster for junior, set junior as the main on screen character. then go to the residential area of kukai and enter the green carpeted room. great joe is hiding in a closet. beat him to unlock the swimsuit and the summon ether GREAT JOE, which can evolve into BUSTER JOE, for Jr. Only.

To unlock more things for Momo, you can fight Mintia. Mintia can be found in two places, but I only know one for now. Set Momo as the main onscreen character, and head to the room on the top of the iron man bar. (it's the room where you can see the first level of the iron man bar from the balcony) Fight Mintia the first time here, to get a Penguin Rod for Momo, and some new ether moves for her.

To fight the Ace Pilot, search the U-Tic simulator, which is the place ziggy is assigned to save somebody in story mode, and look for a room on the north eastern side. In that room is a soldier that will look at you, but won't fight you unless you touch him. He's also blocking a segment door. Beat him in battle and he'll board his mighty AGWS, which I think is a GEAR. The mech is called Meld Gareth. Beat it to get a lot of EXP. early in the game. It has about 5200 HP. Leave the sim and return to fight him again, this time he will be in the Din Gareth, which has about 8200 HP. Using Erde Kaiser on him is the fastest way to beat him. This mech gives lots of EXP. You can always return to the room and fight the same mech (Din) over and over to level up quicker when you reach level 40.