You befriend Jibanyan after getting his picture of Amy back from the Roughraffs. He helps you defeat Dismarelda. The next day, you should meet your friend at Triangle Park.
Meeting in the Park
Go to the park (the same one where you caught bugs on the three trees) and talk to your friend. Your friend had an argument with another friend and decides to go apologize. Whisper thinks it sounds like a Yo-kai might be involved, so you decide to go to the river.
Friendly Enough to Fight
Go west to the river and talk to your friend. Instead of apologizing, they had another argument, and Whisper senses a Yo-kai Inspiriting the student on the middle bench. Use your lens to find it sitting on the upper bench. It's Tattletell. Talk to her.
You get Tattletell's medal, and you convince her to help your friend apologize. Talk to your friend, and they will patch things up.
Tattletell tells you that she saw a businessman leaving the school, and he was saying something about breaking the four seals around town, then she got a sudden energy boost. Whisper thinks that this sounds suspicious. He says that he senses an energy trail leading to the Post Office, so that is where you should go next.
First, you have to go home to rest because it's getting late. When you get up, your Yo-kai Watch will be able to tell time when it's not showing the Radar.
Now you should go to the Post Office. It's in the south part of Uptown Springdale.
Talk to the Shady Businessman, who is standing out in front of the Post Office. Tattletell Inspirits him to make him reveal his secret plans: the businessman is really Tengloom, and he wants to break the four seals around town to become the most powerful Yo-kai.
After his secret is out, he attacks. After you defeat him, he says that his friends are already going to the community center and bank.
Restore the Town Seals
One of the seals is in front of the bank. Check on the piggy bank outside and Whisper will get ready to restore the seal. But then Peckpocket shows up and attacks. After you defeat it and a couple of Leadoni, Whisper fixes the seal.
Another seal is to the right of the community center, which is north of Triangle Park. Check on it and Mochismo attacks, along with two Suspicioni. After you defeat them, Whisper fixes this seal.
The Last Seal
Next, go to the school. Check on the statue near the middle of the field in front of the school. Whisper is about to fix the seal, but Slimamander appears.
Slimamander's vulnerable point is its eye. All three of Slimamanders heads will attack with tongues, but one of the heads also has a hidden eye. You should target a single head until it opens up. If the eye is there, keep targeting that head to do damage to Slimamander. If there is a tongue in that head instead, you should target one of the other heads until you find the eye. Eventually Slimamander closes its heads again, and the location of the eye is randomized after that, so you have to look for the eye again.
This Town's Secret?
After the battle, Slimamander almost hits you with a surprise attack, but then a mysterious figure appears and shields you, defeats Slimamander, then disappears. Whisper wonders why a boss Yo-kai was hiding in town and thinks that this town has secrets.
After that, you go home to rest.
The next day, Whisper says that you need to upgrade your Yo-kai Watch to find stronger Yo-kai. There's a store called Timers & More where you can get it upgraded. They are the ones who created your Yo-kai Watch.