After going down the stairs you'll find yourself in the upper area of Shinsaibashi Station. Head to the right to enter Shinsaibashi Underground. The enemies here are much harder than those in Umeda, but they also yield more EXP. Head to the right. You'll pass an exit marked Danaru (ダナル). Just beyond the exit are some pay phones; simply activate one with 
to ring Kouji and save your game (this is what the Phone Card you got from Miho used is for). If you wish, save your game now. When you are ready head out either this exit or the one further to the right.
Shinsaibashi Street is very short, and unlike in Umeda you can't go up the side streets. If you go to the far left you can find an entrance to another parking garage, but it's guarded by a police officer. If you admit to being one of the "suspicious characters" he's watching out for he'll beat you up. He's unbeatable right now, and you can't go any further at this point in the story. When you are ready to continue the story, head to the right and climb the stairs. Head to the right. If it is the afternoon a cutscene will initiate. If not you'll have to wait around until the right time comes.
It's decided that several Union strongholds will be attacked simultaneously. You're supposed to go into a sewer system accessible through Umeda's subway tunnels. Yoshihiro will give you the Card Key needed to unlock the door. Miho gives you the Sakura Lining and heads back to the hotel. Equip the Lining (unless you've found a rare drop it's better than what you have on right now) and head back to the station. Take the left stairwell this time and either take the train or else leg it back to Umeda. If you want to heal up, go back to the hotel. Miho will be waiting in your room, but she won't come with you. The upcoming boss battle with Hokuto is somewhat difficult, so you may want to save your Ki until you reach her.
Head to the train station and go down the left stairwell. Head along to the right, being careful not to walk on the tracks (trains come by regularly, and you will be killed instantly if it hits you). Along the way you'll pass a door; this isn't the actual entrance, so unless you want a pipe to bash people with you can ignore it for now. Continue to the right until you see a strange recess in the wall. Push 
here to enter the Union assembly point. Five members greet you. This area is actually fairly easy to fight them in. If you stay on the platform and only attack those who climb up to fight you the others will often waste their time jumping in and out of the pit area in an attempt to come up and attack you from behind. This means you often won't have to fight more than one or two who get into the correct position. When only two remain you might as well jump into the pit and finish them off, because when you are on the bottom level they won't spend as much time jumping onto the upper level. Continue to the left. You'll likely face some random battles along the way. Repeat the same process to defeat them. Continue left and enter the door to the Sewer Second Basement. If you want a chain to fight with enter the far right door, otherwise enter the middle door to reach the Sewer Third Basement.
Right after you enter another group of five thugs will attack you. Other than the addition of water, the strategy here is the same. You'll earn a nice 259 EXP. When you're ready, head to the right to initiate the next cutscene. Here you'll fight yet another group of five enemies. You'll get a nice 215 EXP. Now the screen will scroll to the right to show Hokuto.
Being the first real boss you face, Hokuto has a large health bar. Unlike some games, the health bars for enemies do not rescale based on how much health they have. Since she has more than the amount it takes to fill a health bar, it will initially look like you are dealing no damage at all, but in fact you are damaging the invisible portion of her health. Spend your Ki on Strike during this battle, as it will make things a lot easier. Eventually her base health will start being damaged, and soon it's all over. You get 500 EXP for defeating her, as well as the Boxing Gloves and Granny Sandals.
After this Urabe will come out of the door, shriek, and disappear again. The gear you got from Hokuto is far better than what you have right now, so equip them right away. Head through the door Urabe disappeared through. It turns out he got captured while searching the sewers. You must initiate this scene in order for the story to progress. After he leaves, work your way back to the hotel.
Back at the hotel, Miho will ask you to take her back to Shinsaibashi. You don't have to do this right away, however; like all friends, Miho is invincible; you can explore the city with her in tow and she'll gladly help you beat up anyone who accosts you, and this won't have any effect on the story. Note that there is no friendly fire setting, so her attacks can damage you. Since she's invincible you can slam right through her to hit enemies without worrying about her health. You don't get any experience from enemies she kills, but she can help some away while you fight others. When you're ready to continue the storyline head back to the bridge—if you've forgotten the route is to go to the train station, take the right stairwell, ride the train to its first destination, go up the stairs and to the right, out the door and up the stairs to the bridge.
Meetings at the bridge only ever take place during the afternoon; if you arrive at the bridge at night or in the early morning you'll have to wait around until the afternoon. Urabe will turn up and explain what happened to him. Oumi and Saori will run up breathless and explain that the Union knew about the plan and ambushed them, and has captured Yoshihiro and Izawa. Urabe says he has a key that can access the part of the sewer where they are being held. Just then a siren sounds; the policeman you may have come across if you entered the Shinsaibashi parking garage earlier, arrives and tries to arrest you. Urabe and Saori hold him down while Kunio and Miho run off to save their friends.
If you try to go into either station entrance the police will be there, so head to the far left and Urabe will tell you the garage isn't being watched. Now with two friends in tow, head into the garage. Inside you'll overhear Union members discussing how they were the ones who called the police, and then three of them will come out to fight you. That coward Urabe will run off at this point, so it's down to you and Miho to take them out. Urabe now returns. Touyama has returned to his post by this point so the only option is to head into the sewers.
Inside the first room you'll face three thugs. Head through the door to the right and another three will attack you. The door they came through is a dead end, so go through the door to the left instead to jump straight to the Fifth Basement. Drop down into the water and wade to the right. Urabe will try to unlock the door and asks for Miho's help. He then grabs hold of her and some new enemies come out of the door. It turns out Urabe was an inside man. He disappears through the door with Miho and leaves his buddies to take you out. The tall guy in grey is an enemy type you are going to be seeing a lot of. They are much stronger than the enemies you've faced so far. They have full health bars, and love jump kicks. You'll get 381 EXP and be told that Miho will be in Nanba.
Now you have to find your way through the confusing maze of sewer rooms. Head through the door Urabe went through and walk to the right. Ignore the first door you pass and go through the second. Head to the right and go through the door. Head through the door to the right. Head to the left and through the first door you come to (it's marked with a wooden sword). Head to the left and through the door. Head to the right and through the door. Head to the left and through the door. Go through the door and you'll arrive in Nanba.