Final Fantasy XIII-2 has an updated Crystarium character growth system, but it's not being used exclusively for Noel and Serah. Weekly Famitsu's report on the game this week confirms that the monsters you add to your party are also built up using a Crystarium system.
According to a summary of Famitsu's article at Sokuho@Hokanko, the monster Crystarium is similar to the playable character Crystarium. However, in the case of the monsters, you don't use use CP (experience) to traverse the Crystarium chart. Instead, you use items, which you buy from shops or earn after winning battle.
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Back to the monsters, Famitsu confirms that you can edit not only their physical appearance through vanity accessories like sunglasses, but you can also give them names.
Outside of the Crystarium, monsters can also grow through inheritance. By sacrificing a monster, you can make your other monsters inherit its abilities.
There are four general types of abilities that can be inherited: command ability, auto ability, special/characteristic ability (特性アビリティ) and resistance (耐性). (I've included the Japanese names for some of these as I'm not of their English translations). Command abilities and auto abilities can only be inherited by monsters that have the same role as the monster that's being sacrificed.
The inheriting monster won't just gain the same set of abilities as the parent monster. In the case of inheriting to a different role monster, you may get completely new abilities.
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