Merging

In the Spirits menu, the left-most tab is Merge. Here, you can strengthen a spirit by merging other spirits into it.

The base cost to merge is 100 spirit points per enhancer spirit plus an additional 10 spirit points for each level of the base spirit minus 1. So, if your base spirit is level 15, it will cost an additional 140 spirit points per enhancer spirit to merge spirits into this base spirit.

Experience gained from merging is imprecisely represented by a percentage bar.

Merging spirits of the same element results in additional experience gain. Furthermore, merging the same spirits into each other can result in bonus stats. Last, if a spirit has reach maximum level, you can merge the same spirit into it to increase its maximum level.

There are a number of spirits created specifically for merging purposes. These are the Jack, Queen, King, and Joker of each element. These spirits are always at max level. The Jack, Queen, and King provide a large experience boost, and the Joker can be used to increase a spirit's maximum level as though it were the exact same spirit.

"For Merging" Spirits
Some spirits say "For merging" in parentheses next to the name, namely the Prince, Queen, King, and Joker of (element) spirits. These are meant to be used only for merging and have poor battle capabilities as a result. The Prince, Queen, and King provide large amounts of experience when merged, and the Joker is used for limit breaking, which is explained below.

Limit Break
When a spirit reaches its current maximum level, you can still choose it as a base for merging, and there is one slot available to merge a spirit into it. The two options you have here are either another spirit with the same name or a Joker of the matching element. Completing this merge will raise the maximum level of your spirit by 10.

Unconfirmed: It appears that the rarity of the spirit determines how many times you can limit break it. Commons can be done once, uncommons twice, rares three times, and super rares four times.

Random Notes

 * Same color feeding
 * Feeding a lvl 1 common to a lvl 19 rare resulted in 22% gain.
 * Feeding a lvl 1 uncommon to a lvl 19 rare resulted in 44% gain.
 * Feeding a lvl 1 common to a lvl 7 uncommon resulted in 48% gain.
 * Feeding a lvl 1 common to a lvl 5 uncommon resulted in 69% gain.
 * Off-color feeding
 * Feeding a lvl 1 common to a lvl 19 rare resulted in 13% gain.
 * Feeding a lvl 1 common to a lvl 7 uncommon resulted in 29% gain.
 * Feeding a lvl 1 common to a lvl 2 super rare resulted in 71% gain, then 61% -> 33% into lvl 3.
 * Feeding a lvl 1 common to a lvl 1 super rare resulted in 100% gain.