Nighty

Nighty, aka the First, is a pretty much the champion. In the first game, he appears in his normal form at Round 2, later on at Round 50, you could realized something happening to him. On his Champ round, he is UNLEASHED.

In the second game, he is pronounced "Black Stealth", and his new moves is basically sword slicing. He appears in Tweak Skies, at Round 39. He also, in the final round, appears in his SUPA-DUPA unleashed mode.

In the third game, he appears the same thing; unleashed.

Series 1
In round 2, his attacks is combo-related.

In round 50, he appears as a boss, and this time he has more anime-like attacks, like cloning, throwing rocks which split in fourths, or maybe even Kamehaha!

In his champ round, he uses his same attacks, however when you approach him, he approaches you different by using "Paint-related" attacks.

Series 2
He uses the same attacks, but instead he gains 2 new attacks, which are hand rotation and blade slamming.

In the final round, he uses all his brothers and sisters souls to beat you in the TRUE LAST ROUND. He gains alot of new attacks, like throwing Fright-nados, or anything else. Once beaten, he shouts that you can't be the "Eternal" champion, loses all his might, and all his brothers and sisters return to the right place, and the Nightmare Ghoul is released and destroyed by Nighty. That ends the whole series. But...

The Prequel/Series 0
Basically, he appears as a true main character trying to make the lonely green planet no longer lonely.

Trivia

 * Basically, his brothers & sisters are actually a REFERENCE to Terraria's aspects.