Setting your party size to 3 after getting Mr.Saturn in chapter 1 will give you Grayface as a party member, he has 450 hp, 0 pp, 341 strength, 434 defense, 332 IQ, and 101 speed, he has a Heavy Bazooka, a High-strung suit, a Red Cap, and Shades equipped. He works fine in battle, this shows he might've been intended as a hero. He also lasts through the scene where you get defeated by Giegue, and then if you set the party size back to 3 during the seuquence, you'll have 3 heroes already, 2 of which are Grayface, You also can't modify his gear
Killing Sherif Gold-bloom at the 1st boss encounter of chapter 1 will abruptly end the fight, he has 500 hp.
Setting your party size above 5 will simply crash, not very interesting.