Jenn's Brother is the brother of Jenn the Locksmith, a prisoner and enslaved locksmith of the current Lion King's Castle. He will only appear in El Nath after he is rescued
At level 115, you will get a light bulb to the left of your screen with the quest 'The Adventure of Jenn the Locksmith.' You have to use Jenn's spare emergency key to go to the Roof of the First Tower and eliminate the Red Crockys.
The Red Crockys give no EXP and they have the same amount of HP and damage as regular Crocky the Gatekeepers.
You have a 10 minute time limit to rescue Jenn's brother. If you fail to make it in time, forfeit the quest and start it again to get a new (well, broken and old) key.