I got fev ideas (nothing personal to frogboy but too hilarius to miss)
Level 1
Frogboy start in frog like form (say half size of oak). He got very low HP and only one ranged attack, most of the times he avoid fights, unless it is 3v1. He can damage enemies with his tongue and even have chance to paralyze them so they cant fight back.
Level 5
Frogboy understands that he is diffrent then others. He realize that he is something else. He morphs from frog into human with little side effects (green skin, large and round eyes, over sized tongue) (He also recives little one handed sword and gains extra 50dmg)
Level 10
Frogboy prince realizes he is ment to become ruler of Demigod world. He is no longer afraid to confront his enemies and gains active ability "Mass silence", which can silence his enemies everywhere on map. (His swords become larger, it is now bastard sword and gains extra 100dmg)
Level 15
Frogboy prince gains ultimate power, making people wanna follow him and die for him. Every time he sees enemy creep wave there is 33% chance that they will turn back and fight for him. (His sword grows even bigger, becomes now Claymore sword and gains extra 200 dmg)