Student or Learner
Here again, however, Mr. Pond was careful not to ignore or entirely despise the warning; he kept an eye on the German governess, and even engaged that learned lady in talk upon some trivial pretext-- if anything she touched could remain trivial.
What is meaning of "and even engaged that learned lady in talk upon some trivial pretext"?
"to engage the learned lady in talk" simply means to talk to her.
"upon some trivial pretext" means "at some minor opportunity that presented itself." Commenting on the weather or the price of tea would be trivial pretexts upon which to engage someone in conversation.
He often took such minor opportunities to talk to her so that he could keep an eye on her.