Source

File: wp-includes/class-wp-roles.php

public function add_role( $role, $display_name, $capabilities = array() ) {
		if ( empty( $role ) || isset( $this->roles[ $role ] ) ) {
			return;
		}

		$this->roles[ $role ] = array(
			'name'         => $display_name,
			'capabilities' => $capabilities,
		);
		if ( $this->use_db ) {
			update_option( $this->role_key, $this->roles );
		}
		$this->role_objects[ $role ] = new WP_Role( $role, $capabilities );
		$this->role_names[ $role ]   = $display_name;
		return $this->role_objects[ $role ];
	}