Do you change at all based on who is closest to you at the time?

Yes and no, I guess?
There is a lot of me. A lot of things I enjoy, a lot of ways I act.
If I’m spending a lot of time with a certain friend, what I end up doing would be things I know we both enjoy, or something they enjoy that I’ll participate in because I love them.
That changes slightly with everyone I’m close with. There are people I hysterically giggle with, and people I’m more calm with, but both are parts of me.
So being around someone doesn’t change in that I’m a new person when I’m around them. But when I spend time with someone I’m sure I’m subtly changing in a way that would be different if I was alone or spending time with someone else.

The things I DO certainly change depending on who I am dating or to whom I am married. The second ex, for example, was an outdoors person and loved fishing, boating, camping and hiking. I LEARNED to love those things also, because I had never been exposed to them before.

The third ex loved to hang out in bars, smoke pot, snort coke, play pool and video games...and again...I had never been exposed to that before. I can't say I didn't LIKE to party hard, but it certainly isn't good for you.

Yes. In past relationships, I might feel anxious, insecure, empty, and even depressed.
In my current relationship, I feel secure, serene, fulfilled, loved and happy. ☺💕

A little bit. It’s kind of hard not to change a little especially if they are close to me.

I do have hobbies which I enjoy and sometimes if I find myself change too much such that I start to neglect my hobbies, I then need to take a step back and find my own space again.

Hmmm no.
Not much anyway.
I mean, yeah slightly, and I do consider the person I'm with. But I don't change entirely.

I mean I'm all for trying new things, but I have my likes and dislikes. Most of my close friends have similar or compatible lifestyles, hobbies, etc.

If your with a very tense person in a relationship they'd make you tense .
If your with a happy person in a relationship they"all make you happy .
Simple as that .
I had a friend and when I was hanging around with him he'd put tension into you and you didn't have to say a word

I do alter my demeanor depending upon whom I am with.

Yeah, I definitely do. I like to fit in.

Maybe,

People are influenced by their peers .. some more than others