Also about his birthmark, why is it not there when the gu guy brought him to the bar, but it's suddenly there when he's revealed to be the son of that annoying family? Is this an artist error. The art is so good though.
I think MC is prodigy in terms of business or anything involves money but he has no capital, like look at his bio parents their loaded so they must be professional money maker and he inherited that trait, when he became a substitute there must be money involved in the contract and he use the money he gain from assh*le and try to secretly gather intel and people for his business venture, we don't know cause this is fiction.
about the birthmark, well yes it was inconsistent, like lu menchin's scar on the neck the second it was there but now in the next page it's gone, it depends on the artist and author.
It's better explained in the novel. He already set up his company before he became a substitute. He had good business acumen, so his investments or project selection was supposedly good and during the time he acted like a substitute. It went from a no-name company to a black horse in the industry. The Gu guy would give him money to use as "allowance" and mc basically invested all of it in the company. I think he also picked up on news and dealings while he was with the Gu guy because him and his friends didn't really give him much thought.
Is it just me wondering how the mc suddenly became rich? Like he was struggling to pay his mom's hospital bills and his dad's also in jail. So he willingly became a substitute to get money. But when the ml came, I get that the ml is rich, but how did mc suddenly become rich too? Did the ml give his money or did the mc do stocks or what???