I won't go into all the requirements for a person to qualify as a dependent, but since there is no legal relationship, a dependent must live in your household (home) for the entire year. Another requirement to qualify as a dependent is that they be a US citizen, resident alien, or national.
The IRS also has this webpage (only if the person meets the second requirement above) for determining if a person qualifies as a dependent.
https://www.irs.gov/help/ita/whom-may-i-claim-as-a-dependent
