During his travels, Jiraiya met Kakashi in an alleyway once.
The young man was crouching behind some boxes. Jiraiya approached him, intent on striking up a conversation. Kakashi did not acknowledge his presence, too focussed on whatever he held in dirty, bloodied hands.
It was a dog.
Jiraiya asked, "Do you like dogs?"
Kakashi asked, "Why do people hurt dogs?"
Jiraiya didn't know, but he stated: "To test their loyalty."
Kakashi nodded in thoughtful agreement, eyes dark. Jiraiya ruffled the dog's grey hair, "Let's get you patched up," and he took the Anbu-mask from his hands.