Byonko is a talking frog mamodo from the Mamodo world. Byonko himself gets upset very easily and is always worried especially about making friends. During battle he is shown to be complimenting himself on how good his skills are instead of worring about the enemy.
Byonko's abilities are mainly slime based which matches up with his frog personality perfectly. It mostly has to do with stretching his limbs or shooting out bubbles made out of slime. He is also shown to be extremley strong being able to knock down Demolt.
Spells
Gigano Ribbisto: An unknown spell that Byonko attempted to use to save Penny from Zatch's Bao Zakeruga. However, Byonko was unable to use it since Alvin was not present. (Attack)
Gigaro Nyururuk: Byonko's hands and feet will stretch to an incredible size. (Attack)
Raja Nyurusen: Creates numerous, slippery, green bubbles that make an enemy slip and fall. Used to prevent enemy from counterattacking. (Immobilization)
Giga Nyushield: A green shield made of slime in the shape of a crown. (Defense)
Shin Nyushield: Byonko's strongest spell, seen in Zatch's golden spellbook. A slimy membrane that is formed around a spell in order to reinforce its strength. (Assist)
Source: ZatchBell Wiki