That's why they always warn you even in a country full of laws to becareful when signing as someone's guarantor.
But did he really sign something like that?
Based on Chapter 2, MC said he doesn't remember signing anything like that. Then we get chapter 4 flashback revealing that his dad secretly had MC's fingerprint put on the loan papers without consent.
Although this is illegal and can not count as MC giving consent, MC would have to go to court to prove that it wasn't him who signed. Setting up court dates, lawyers, etc would've exhausted all his time and money. Plus court dates usually take months to get. And until this is approved, the loan technically falls onto mc and the document is legally binding until proven false.
It's also illegal in most countries to harass people to repay their debts, especially unrelated fsmily members. HOWEVER, whether these laws are enforced or not depends on the authorities.
Why the hell is that even possible, like how can they still pursue the guarantor if the original debtor is present???!!!! Shitty situation that is