Hi
@saulot! I'm gonna nudge this over to Help & Q/A since your thread is worded like a technical question and people are answering it as such
The penalty for being a lonely kitten is small, giving -1 Bravery and Extroversion once they grow up into a Young Kitten. This won't be particularly noticeable unless your kitten is already low on the Bravery and Extroversion scale; for example, if they turn out Adventurous, you likely wouldn't notice it!
You have two options: have a litter of beans and adopt your lonely bean into it (they don't have to have the same parents) or hope that Nestor gives you another bean that could become their adoptive littermate!
As a fun fact, every Young Kitten and Kitten from Nestor is generated with the lonely kitten penalty already, as the adoptive littermate option is only available to beans.
Good luck with your bean! Whether or not they end up getting a littermate, I'm sure they will end up with lots of love from the other village members