Diablo is as he appears at the end of Diablo 2, with all of his various attacks.
Albert Grabner and his Wanzer, the X-D "Dragoon", are as seen in Front Mission: Gun Hazard. The Dragoon has the best shield, dash, and vernier units equipped (SHD-04, RD-400, and VU-50 respectively), and its primary weapon is VG-05. Secondary equipment on it includes an EAP-50 (Armor Plate), HNP-50 (Hard Knuckle), MIN-50/FBG-50/DCY-50/HMS-50. Grabner himself is fully equipped with his jetpack, HAV-05, HGR-05, and HHG-05 (armor, grenades, and handgun).
Agahnim has all of his abilities displayed from his endgame fight in LttP, where he gets two shadow clones. He can be damaged by other means than reflecting his magic back at him.
Roo is as in his playable form from Streets of Rage 3. The opposing team will not attack Roo until he first attacks someone on the opposing team, or until Roo is the last remaining fighter on his team. Also, Gene has attached a God Hand to Roo. Roo will know how to use the arm naturally, and will also be able to enter his God Hand state once in the fight for 15 seconds, and no he isn't invincible then either. The only Roulette Wheels he has available to him are Shockwave, Kung Fu Samba, God Stomp, Divine Smash, Daisy Cutter, and Typhoon Kick.
Gene is as he appears in God Hand, with both God and Devil Hands. He has access to all his techniques, unlimited access to Level 1 Roulette Wheels, and has a stock of 6 Wheels for higher level ones. Though Gene enters the battle with the God Hand trigger ready, he may enter it only 3 times for the battle. He's never invincible (e.g., God Hand, dodging) and does not have access to Chain Yanker. He has turned into a fluffy bunny.
Cielo is as he appears in Digital Devil Saga 2. Regarding spells and skills, he has all his gun skills, Void spells, elementals up to Ma_dyne, Xanadu, Wicked Curse, non-revival healing, non-ID Hunt skills, Chi Wave, Power Wave, Executioner, Megido, Megidola, and all buffs and debuffs. The only passive skills he has are Fire Resist, Ice Resist, and Expel Resist. He may freely turn into any of his 3 forms - demon, human, or the hybrid.
Athos is as seen in Fire Emblem: Blazing Sword, a Level 20/20 Archsage. He has Forblaze, Luna, Gespesnt, Excalibur, Fortify, and Bolting. In addition, he knows and is a master of every magic spell possessed by all his allied fighters participating in the battle, though only of their base forms, before abilities or transformations take effect.