Triparty agent
One who manages a tri-party collateral system. A custodian who opens separate accounts in the name of a borrower and lender, and transfers collateral between them at their joint instruction, in satisfaction of margin obligations under a separate master agreement.
Very popular with agent lending participants as a handy way of managing collateral flows, as this handy diagram purports to illustrate.
Can be a useful fellow if you want to grant security over a revolving pool of securities and retain a right of substitution. Some useful discussion there at Financial Collateral Regulations.