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 cars such as the Turismo R, Zentorno or T20 as regular cars, and will just say regular quotes such as "What brings you in today?", not the ones designated for vehicles like the Adder, Entity XF or Cheetah such as "You've got hyper quality."
This is most likely a developer oversight, since these vehicles were later added in-game through DLC updates.
In single player, if the player drives into the garage but promptly exits without purchasing anything, the mechanic will say "Did you need something or are you just being a dick?"