Greg Kroah-Hartman is known as second-in-command of the Linux kernel. The ‘gentle giant’ is the maintainer of the stable branch of the kernel and of staging subsystem, USB, driver core, debugfs, kref, kobject, and the sysfs kernel subsystems along with many other components of a Linux system.
He is also credited for device drivers for Linux. One of his jobs is to travel around the globe, meet hardware makers and persuade them to make their drivers available for Linux. The next time you plug some random USB device to your system and it works out of the box, thank Kroah-Hartman. (Don't thank the distro. Some distros try to take credit for the work Kroah-Hartman or the Linux kernel did.)
Kroah-Hartman previously worked for Novell and then joined the Linux Foundation as a fellow, alongside Linus Torvalds.
Kroah-Hartman is the total opposite of Linus and never rants (at least publicly). One time there was some ripple was when he stated that Canonical doesn’t contribute much to the Linux kernel.
On a personal level, Kroah-Hartman is extremely helpful to new developers and users and is easily accessible.