Basics here : http://ag.igg.com/playercenter/news_view.php?aid=55
Not sure about crystals but info about them here: http://ag.igg.com/playercenter/news_view.php?aid=56
Edit - crystals are for passive abbilities
| 4 Big hearts | pet share 5% of DEF and MDEF | when you use ability it's exp increase by 2 points | pet can learn 6 passive skills |
| 5 Big hearts | 5% pet share 5% of Attack and SPE | skills increase by 3 each use | pet can use accesories |
| 6 Big hearts | 5% pet share of HP and MP | skill get 4 points eachuse | pet can learn 7 passive skills |
| 7 Big hearts | 5% of Sodge hit rate and speed | 5 points each use | pet can use 2 of something(probably crystals) |
Not sure about crystals but info about them here: http://ag.igg.com/playercenter/news_view.php?aid=56
Edit - crystals are for passive abbilities
