Head to the far left. Along the way you may pass some Tigers fans. They'll tell you Masaki is in the archives. It's the building right at the very left of Koushien. The letter explains Miho's capture and indicates that Masaki should work against the Union to save his sister. Masaki explains more about why the Union became obsessed with ruling the city. He gives you the Bike Key, which allows you to access any major area of the city at will—you never need brave the subway stations again, and Taxi Cards become essentially useless.
Back outside some thugs will say they overheard it all and that Masaki will be in trouble. They also say your friends at the hotel are probably getting whacked by Yoshihiro, who it seems is another inside man. After this they'll attack. When they're dead use the Bike Key to get back to Umeda. When you get back to your floor you'll find Takaaki almost unconscious on the floor. The two guys responsible will come out and say it's done as a warning. Your other friends will then come in, and you'll get into a big fight with the thugs. Be careful here, as your friends won't hesitate to punch you in the neck while trying to hit a thug you've grabbed. You'll get 670 EXP when the battle is over. Your friends will tell you to get revenge, so off you go. You'll end up on B1. If you want to heal up first, head back to your room and have a rest. Saving is also a very good idea. When you're ready, use the Bike Key to go to Nanba.
When you arrive at Nanba Ebisubashi Street three punks you've previously defeated will attack you. At this point in the game they're probably no trouble at all. Head to the right. Noboru will be standing guard and says you've come just as Yoshihiro said you would. Wada arrives and taunts you, then Noboru and four other Union members will attack. This is the largest battle in the whole game. Enemies just keep coming. Once you've hospitalized a good 50 or so Union members, there's yet more fun to be had. Past bosses Nakazaki, Hokuto, Urabe, Yasuyuki, and Wada all show up to send you to the afterlife. It will probably come down to you and Wada because of his insane amount of health. Remember to equip the Power Wrist (if you have it) the moment you lay the killing blow. You'll now receive an incredible 13,336 EXP, or the even more amazing 26,672 if you are wearing the Power Wrist. Unless you've leveled up a large amount already you should get at least one level out of this battle.
After it's over Kizaki, one of Masaki's men, tells you the Union is going to attack Ebisuchou tonight. Use the Bike Key to return to the hotel and rest up, then go to Ebisuchou.