Soul can be strengthened to achieve better attributes.
The "+" indicator on the Soul represents its level. Soul begin at level 0 and can be strengthened to level 15. Soul must be strengthened by sacrificing other Soul (where that Soul's experience is partially inherited) and they cannot level past the level cap, nor can they be ranked up to increase star rating. Strengthening also requires gold. The cost to strengthening depends on the stars of the Soul.
For secondary attribute chances when strengthening Soul to levels 3/6/9/12/15:
- If Soul has 4 secondary attributes, an existing secondary attribute will be randomly raised. Every attribute has 1/4 chance of being raised. Otherwise, existing substats have 9% chance, and new substat chance is 100%-9%*number of substats.
- If Soul's secondary attribute does not reach 4, then a sub-attribute is randomly added. Add chances are: attack-type 36%, defense-type 36%, utility-type 28%. (Includes attributes that already exist, it is possible to not get a new one)
Soul require more experience to strengthen as their grade rises.
Soul at different grades and levels have different amount of gold requirements and experience yield. At level 0:
The other levels are too hard to find out as experience yield varies even with the same level - mitama could have their experience bars filled to varying degrees. What is known, at least, is that experience is not inherited in full.