PuppyChung: His eyes have paw prints for pupils and his hair looks like puppy ears.
PikaChung: Both ends of his hair have a patch that resembles a certain Pokemon's ears. Once the Shelling Guardian/Tactical Trooper class line came out, this was applyed to them due to how those bits stick up like ears.
Chungdam: His futuristic armors and his cannon has led to him being mistaken for a Gundam. Especially as Tactical Trooper.
Chungina: A female variation of Chung, used due to frequently being mistaken for a girl, on account of his young, and overly cute looks. This nickname began to fade away as he takes on Shooting Guardian route, but is further enhanced as Fury Guardian. Greatly enhanced during the 2011 April Fool's Day event in Korean version, when all characters have been Gender Swapped.
Chung's Iron Paladin promotion (upgraded from Fury Guardian) further enforces it; his hair is down to his rear, and his armor gains three inch heels. Some of the promotional art for this form has a distinctly hourglass-ish waist, too.
Iron Princess: Chung's Iron Paladin class is called this, due to the above.
Elboy: The fandom's Affectionate Nickname for the titular character (and to disambiguate him from the title itself).
What Could Have Been: During development, the character Elsword was originally meant to be related with Elesis from Grand Chase, and Aisha and Rena were originally named Arme and Lire respectively. However, due the copyright reasons and Elsword and Grand Chase being hosted by different companies, this idea was scrapped.
However, due to Elesis' introduction in the Korean server, and her being the same person as Elsa, Elsword himself gains his official surname: Sieghart.