I believe the correct word would be misanthrope or misanthropist.
As @theUg suggests, I am elaborating my explanation. Following are the reasons why I believe misanthrope should be the correct word here:
Misanthropy is the general hatred, mistrust or disdain of the human species or human nature. A misanthrope, or misanthropist is someone who holds such view or feeling. The word's origin is from Greek words μῖσος (misos, "hatred") and ἄνθρωπος (anthrōpos, "man, human").
- Scrooge as @JAM suggests could also be considered correct but not 100%, because the dictionary defines it as a person who is miserly. Also misanthrope looks much more formal. Same is the case with words suggested by others as they are not 100% correct for this situation.
Spying, is the act of obtaining secret or confidential information without the permission of the holder of the information. The person who provide this service for fee is called private detective.
Private detective
They collect useful information from various sources, then they provide it to the client for a fee. They are normally hired to do specific job of collecting information of the targeted person.
On the other hand, these private detectives may take help of Data Brokers.
Data Broker: Excerpt from wiki article explains it in detail:
An data broker or information broker collects information about individuals from public records and private sources including census and change of address records, motor vehicle and driving records, user-contributed material to social networking sites, media and court reports, voter registration lists, consumer purchase histories, most-wanted lists and terrorist watch lists, bank card transaction records, health care authorities, and web browsing histories.
Your sentence,
If you're looking for someone who knows a lot about others, he's the right person, he's private detective.
Though, they don't have information of the targeted person before hand. After you hire them, they collect information of the targeted person for you.
Hope it is helpful!
Best Answer
We can call that person a homebody.