Who was the sentry at the entrance of the rosebush in the Rats of NIMH?

In "Mrs. Frisby and the Rats of NIMH," the sentry at the entrance of the rosebush is a rat named Brutus. He is tasked with guarding the entrance to the rats' home and plays a role in protecting the community. Brutus is depicted as strong and vigilant, ensuring that no threats disturb their sanctuary.