To play as Mr. Phatt:
While fighting Mr. Phatt, finish him off with a combo that does 
at least 48 points of damage. Once he's unlocked, you can select 
Mr. Phatt on the Character Selection screen.

To play as Abaddon: 
Complete the entire game in Captain difficulty mode. Once he's 
unlocked, you can select Abaddon on the Character Selection screen.