The familiar fostering system allows you to choose up to three familiars and have them level up simultaneously at the farm. For the cost of some gold, players can leave up to three familiar at their farm to gain EXP at a 30% rate.
Speak with NPC Lumein to register familiars you’d like to have participate in Familiar Fostering. There is a total of 3 slots overall. The 1st slot is FREE to use. The 2nd slot costs 30 Ellun 
and the 3rd slot costs 100 Ellun .
You can increase familiar EXP gain even further by giving each registered familiar some “feed”. You can choose between Budget feed and Premium feed. Budget feed will allow the familiars to gain EXP at a 60% rate and can be purchased in a 3 hour increment with Familiar Coins. Premium feed will allow familiars to gain EXP at a 120% rate and can be purchased in 1, 6 or 12 hour increments. The 1-hour version can be purchased with Familiar Coins.
You can also increase the EXP rate even more by upgrading your farm. A mid-grade stable provides an increase of 50% experience gain while a high-grade stable will yield an increase of 100% experience gain. The experience is only applied when you are logged into the game.