Further more in the next verse BGAII 10.9, Srila Prabhupada gives a clear definition of a “pure devotee.”

“Pure devotees are those who thoroughly understand the Supreme Personality, become fully engaged in His service, and talk about Him, and preach about Him and thus are satisfied and enjoy spiritual bliss.” The Challenge Bhagavad Gita As It Is 10.9

So from this verse we see that there are 6 qualifying elements to a pure devotee as follows:

ONE. His thorough knowledge of Krishna

TWO. permanently 24/24 and 365/365 engages in Krishna service

THREE. They talk amongst themselves about Krishna.

FOUR. He / She is a preacher.

FIVE. A satisfied individual

SIX. Enjoys spiritual bliss