One moment. The person who Zilean is based off of Zileas, thought the idea of a bear champion bring brought to the League was stupid. The office was jokingly divided into two sides, Volibar's side who wanted an armored bear, vs Zileas who didn't. As you know, Volibear WAS released so as a joke, they both have hidden passives where they are against each other in game. (Does not work if they are allies). Zilean gets "Armored Bear Hater" which is a picture of Volibear's passive crossed out while Volibear gets the Chrono Keeper Hater which is Zilean's passive crossed out. If Zilean kills Volibear, he gets +10 gold. If Volibear kills Zilean he gets +11 gold and says one of two quotes: "Your time has run out, Chronokeeper" or "I got a special claw for you, Zilean."
Here is a video, I hope that clears everything up.