Any person not related to you by blood can only be a "qualified relative" dependent. To be considered a qualifying relative dependent, the person must live as a member of your household for the entire year, you must provide more than half their support, and they can't have more than $4400 of taxable income. This is a $500 credit.
The person does not need a social security number. As a non-resident alien, they would apply for an ITIN (international taxpayer ID number). They can't apply for an ITIN now, the application must be mailed together with the tax return which claims them as a dependent (and since you have to mail the return and application, you can't e-file.)
