The Mechanic is generally friendly, although may be short with the character if they drive in and out, asking "What's wrong with her now?" He may also say this when the player drives in. The mechanic, like Hao, can be killed, but will respawn.
The Mechanic treats downloadable-content cars such as the Turismo R, Zentorno and T20 as regular cars, and will say regular quotes such as "What brings you in today?", rather than the ones designated for vehicles implemented before any DLC ever came out, like the Adder, Entity XF or Cheetah.
In single player, if the player drives into the garage but promptly exits without purchasing anything, the mechanic will say something like, "Did you need something or are you just being a dick?"