Ancestral Mech
Picking the Ancestral Mech background option will trigger an event a day or two into your career that adds a light 'mech to your mechbay. The exact 'mech in question differs based on which option you picked in the "Before Becoming a mercenary, you came from..." field. The following is the list of the 'mechs provided with each origin.
The Magistracy of Canopus: Locust LCT-1E
The Federated Suns (Davion): Valkyrie VLK-QA
The Draconis Combine (Kurita): Panther PNT-9R
The Capellan Confederation (Liao): Firebee FRB-2E
The Rimward Periphery: UrbanMech UM-R90
The Free Worlds League (Marik): Spider SDR-5V
The Deep Periphery: Crosscut ED-X2
The Lyran Commonwealth (Steiner): Commando COM-2D
The Taurian Concordat: Toro TR-A-6
The Outworlds Alliance: Mongoose MON-67
The Free Rasalhague Republic: Hussar HSR-300-D