Tron Bonne from Mega Man Legends is in Project X Zone as a solo character. You can call her into battle to use King Servbot, which summons a rush of curry carrying Servbots before transforming into a giant Servbot.
Lady from Devil May Cry attacks with the Kalina Ann rocket launcher and Bahn from Fighting Vipers pummels enemies.
Lady from Devil May Cry
Tron Bonne from Mega Man Legends
Bahn from Fighting Vipers
Lindow Amamiya from God Eater
Reiji and Xiaomu from Namco X Capcom are also playable characters in Project X Zone. These two are a unit you can move around instead of a solo character you can summon.