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Using the Grimoire's Appraisal, I carefully looked at my own Unique Familiars Status while processing the bonuses each one gave to me. Not only did they become overwhelmingly strong after hitting Rank B+, with very high stats all across the board, but they even granted me permanent stat bonuses and a skill they "inherited" to me!
Unlike my siblings I didn't get a Divine Protection though, and it's not like I need it, but that might be because of how I raised them differently. Apparently Unique Familiars grow and shape different according to the initial desire of the former owner and the new desire of their masters.
Because I wanted them to protect my siblings, they developed something as powerful as the Divine Protections, but because I wanted mine to just be strong and useful, they gave me the most basic rewards, while prioritizing their own growth instead.
Ding!