Internally, we each tend to have unique personalities that make us who we are. This would be our overall personality.
Externally, our personalities tend to change depending on our surrounding. We tend to show a different side of ourselves to one person than we may another. We may also vary in personality depending on other factors such as if we are at work, we may become more stiff and goal orientated vs. Away from work we may be much more relaxed and behaving like we are just going with the flow. Our mood or recent changes may also determine our personality. A person who is normally a happy go lucky type of guy may turn into a moody person after the loss of a loved one.
I for one believe we all have numerous sides to our personalities, and we show different sides to different people. There are things I would say or do with my friends that I never would with my family and vice versa. This isn't to say everyone is two-faced; you're not being fake with one person and real with another, you're just revealing a different part of yourself to them respectively. I would say each person has at least four aspects to their personality; when they're with friends, when they're with family, when they're with a significant other and when they're alone. Of course, certain people do change their opinions just to fit in with someone or a group of people- but that's another issue.
Because for an example the poisonous snake lives on the berry tree so... Do the tree become poisonous like the snake or keep yielding berries ? So no it dose not because if you have a good soul and a good heart you will always be like that even if you have a bad soul or heart
Your demeanor does. Your mood may. Your personality is considered to be a more concrete set of behaviors and outlooks so for instance it could be in your personality (rigid) to be in a bad mood around specific members of humanity (let's use Scots for instance) but it isn't your personality that changes if you like being around a specific person in humanity, just your demeanor towards them.
In essence you can hate Scottish people but still like a few Scottish people without changing personality. This is more apparent with dogs and cats though in real life.