In one sentence
The principal or firm behind a real estate deal, whose experience, capital, and track record the lender underwrites alongside the property.
The longer version
The sponsor drives the business plan and often provides the guaranty. Lenders underwrite the sponsor's experience, financial capacity, and track record, because on a non-recourse loan the sponsor's competence is much of the protection.
Sponsor diligence, financials, references, and litigation and background checks, runs parallel to property diligence.
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